How to return a vector from a function

Question

I'm trying to return a vector from a function, but I seem to be having some issues ....

Currently, this is the best I've got:

int* OptimizedSkillLevels(int skillLevel, const int numSkills, int duration, FragmentOptimizationParameters FOP){

        //some code to work with some things was here

        vector<int> skills(numSkills);

        int skillIncrement = 0;
        while (skillLevel > FOP.levelDifference){
        skills[skillIncrement%numSkills] += FOP.levelDifference;
        skillLevel-= FOP.levelDifference;
        skillIncrement++;
        }
        skills[skills.size()-1] += skillLevel;

        //at this point, the vector skills is full of the correct values, which is what really matters.
    }

    int* skillsArr = &skills[0];
    return skillsArr;
}

Though this seems to be giving me garbage values when I access it from this function call:

cout << OptimizedSkillLevels(skillLevels[i], userMaxNumSkills, durations[i], FOPs[FOPindex])[j] << ", ";

specifically, I'm getting -17891602 from that every time, regardless of what [j] is.

So then I tried to convert to a vector as follows:

vector<int> OptimizedSkillLevels(int skillLevel, const int numSkills, int duration, FragmentOptimizationParameters FOP){

        //some code to work with some things was here

        vector<int> skills(numSkills);

        int skillIncrement = 0;
        while (skillLevel > FOP.levelDifference){
        skills[skillIncrement%numSkills] += FOP.levelDifference;
        skillLevel-= FOP.levelDifference;
        skillIncrement++;
        }
        skills[skills.size()-1] += skillLevel;

        //at this point, the vector skills is full of the correct values, which is what really matters.
    }

    return skills;
}

with the function prototype:

vector<int> OptimizedSkillLevels(int skillLevel, int numSkills, int duration, FragmentOptimizationParameters FOP);

But I'm getting a multitude of errors, including this (at the line of the prototype):

1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(59): error C2143: syntax error : missing ';' before '<'
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(59): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int

and these, none of which appeared before changing the return type of my function to vector ... so I really don't know what going on.

    1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(59): error C2143: syntax error : missing ';' before '<'
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(59): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(73): error C2872: 'vector' : ambiguous symbol
1>          could be 'c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(59) : int vector'
1>          or       'c:\program files (x86)\microsoft visual studio 11.0\vc\include\vector(655) : std::vector'
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(73): error C2143: syntax error : missing ';' before '<'
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(73): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(73): error C2086: 'int vector' : redefinition
1>          c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(59) : see declaration of 'vector'
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(74): error C2872: 'vector' : ambiguous symbol
1>          could be 'c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(59) : int vector'
1>          or       'c:\program files (x86)\microsoft visual studio 11.0\vc\include\vector(655) : std::vector'
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(74): error C2143: syntax error : missing ';' before '<'
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(74): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(74): error C2086: 'int vector' : redefinition
1>          c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(59) : see declaration of 'vector'
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(84): error C2065: 'FOPs' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(84): error C2228: left of '.push_back' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(85): error C2065: 'FOPs' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(85): error C2228: left of '.push_back' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(86): error C2065: 'FOPs' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(86): error C2228: left of '.push_back' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(87): error C2065: 'FOPs' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(87): error C2228: left of '.push_back' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(88): error C2065: 'FOPs' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(88): error C2228: left of '.push_back' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(89): error C2065: 'FOPs' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(89): error C2228: left of '.push_back' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(91): error C2065: 'FCPs' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(91): error C2228: left of '.push_back' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(92): error C2065: 'FCPs' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(92): error C2228: left of '.push_back' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(93): error C2065: 'FCPs' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(93): error C2228: left of '.push_back' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(94): error C2065: 'FCPs' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(94): error C2228: left of '.push_back' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(95): error C2065: 'FCPs' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(95): error C2228: left of '.push_back' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(96): error C2065: 'FCPs' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(96): error C2228: left of '.push_back' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(169): warning C4244: 'return' : conversion from 'const double' to '__int64', possible loss of data
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(192): warning C4244: 'argument' : conversion from 'double' to '__int64', possible loss of data
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(195): warning C4244: '=' : conversion from 'double' to 'int', possible loss of data
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(196): warning C4244: '=' : conversion from 'double' to 'int', possible loss of data
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(197): warning C4244: 'argument' : conversion from 'double' to '__int64', possible loss of data
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(202): warning C4244: '+=' : conversion from 'double' to 'int', possible loss of data
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(203): warning C4244: '+=' : conversion from 'double' to 'int', possible loss of data
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(215): error C2872: 'vector' : ambiguous symbol
1>          could be 'c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(59) : int vector'
1>          or       'c:\program files (x86)\microsoft visual studio 11.0\vc\include\vector(655) : std::vector'
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(215): error C2062: type '__int64' unexpected
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(216): error C2872: 'vector' : ambiguous symbol
1>          could be 'c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(59) : int vector'
1>          or       'c:\program files (x86)\microsoft visual studio 11.0\vc\include\vector(655) : std::vector'
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(216): error C2062: type '__int64' unexpected
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(217): error C2872: 'vector' : ambiguous symbol
1>          could be 'c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(59) : int vector'
1>          or       'c:\program files (x86)\microsoft visual studio 11.0\vc\include\vector(655) : std::vector'
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(217): error C2062: type '__int64' unexpected
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(229): error C2065: 'optimalCosts' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(229): error C2228: left of '.size' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(231): error C2065: 'optimalCosts' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(231): error C2228: left of '.resize' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(232): error C2065: 'skillLevels' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(232): error C2228: left of '.resize' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(233): error C2065: 'durations' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(233): error C2228: left of '.resize' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(236): error C2065: 'optimalCosts' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(239): error C2065: 'optimalCosts' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(239): error C2228: left of '.size' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(242): error C2065: 'optimalCosts' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(243): error C2065: 'optimalCosts' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(244): error C2065: 'skillLevels' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(245): error C2065: 'durations' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(251): error C2065: 'optimalCosts' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(251): error C2228: left of '.size' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(252): error C2065: 'optimalCosts' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(252): error C2065: 'skillLevels' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(252): error C2065: 'durations' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(265): error C2872: 'vector' : ambiguous symbol
1>          could be 'c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(59) : int vector'
1>          or       'c:\program files (x86)\microsoft visual studio 11.0\vc\include\vector(655) : std::vector'
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(265): error C2062: type 'int' unexpected
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(266): error C2872: 'vector' : ambiguous symbol
1>          could be 'c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(59) : int vector'
1>          or       'c:\program files (x86)\microsoft visual studio 11.0\vc\include\vector(655) : std::vector'
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(266): error C2062: type 'int' unexpected
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(269): error C2065: 'optimalCosts' : undeclared identifier
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(269): error C2228: left of '.size' must have class/struct/union
1>          type is ''unknown-type''
1>c:\users\vasu\documents\visual studio 11\projects\composing calc\composing calc\composing calc.cpp(269): fatal error C1903: unable to recover from previous error(s); stopping compilation
1>

Any help anyone could offer on either the proper method of returning a vector or some sort of workaround would be much appreciated.

If you want to return a vector, declare your function as returning `vector`, not `int*`. The two are not close to equivalent. — Barmar, Sep 15 '13 at 03:56
The first chunk of code you've got returns the address of a local variable, which is the exact same problem raised in your last question. You need to get some fundamentals squared away. — Chris Hayes, Sep 15 '13 at 03:57
Give [this](http://stackoverflow.com/questions/6441218/can-a-local-variables-memory-be-accessed-outside-its-scope) a good read. — chris, Sep 15 '13 at 04:01

score 5 · Answer 1 · answered Sep 15 '13 at 03:56

5

Your first approach returns a pointer to an object that no longer exists. That's obviously never going to work.

Your second approach is correct. Just fix the syntax errors. Do you have a #include <vector>? Do you have a using namespace std;?

answered Sep 15 '13 at 03:56

David Schwartz

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Actually, I have both of those things you mentioned David. That's why I'm extremely confused as to why I'm getting all of these errors thrown; it _appears_ to be that I'm using all of the right syntax in the second function version, with the vector. – Vasu Sep 15 '13 at 04:04
@BenVoigt, I just did, but from what I read doesn't `using namespace std;` eliminate the need for `std::` in front of code? Could you explain why adding `std::` made a difference (it doesn't return the garbage value anymore, instead it returns 1 and then throws a vector index out of bounds error) – Vasu Sep 15 '13 at 04:14
@Holyhades666: Yes, `using namespace std;` permits the compiler to find its contents without putting `std::` in front... but I have no idea where in your code `using namespace std;` appears or if you've done something weird that would prevent it from working right (e.g. macro named `using`). So I suggest the `std::` prefix anyway. It also helps future readers of your code know for sure which `vector` class you're using. – Ben Voigt Sep 15 '13 at 04:26
1

If I may: even though it's common in typical C++ learning books, `using namespace std;` is generally considered bad practice. You are better off avoiding it from the get go instead of getting used to it. – Marc Claesen Sep 15 '13 at 06:25
Returning a vector by value makes a copy, which is inefficient for large vectors. Make it return a pointer to a vector allocated by new instead. Or pass to the function a vector by reference and full it within the function. – Konstantin Sep 15 '13 at 07:04
@Konstantin It acts as if it makes a copy, but it does not [necessarily actually make a copy](http://en.wikipedia.org/wiki/Return_value_optimization). – David Schwartz Sep 15 '13 at 18:20
Cool, didn't know that. But do you think it's a good idea to rely on the compiler to perform this kind of optimization? – Konstantin Sep 15 '13 at 19:57
@Konstantin It's a great idea because it lets you write clean, clear code that's easy to understand and maintain. – David Schwartz Sep 15 '13 at 20:38

How to return a vector from a function

1 Answers1