Can't figure out how to pass struct array to function

Question

I have a header file, main cpp file and functions file and am trying to pass a stuct array from the main function to the functions file and can't figure out the syntax. I have stripped the code done to show what I am doing without showing hundreds of lines of code.

I have tried every variation i know of to pass the structure array and it won't compile; I have read everything I could find on it and can't find the solution; I know it is just a syntax thing I'm not figuring out.

header.h file

#ifndef myHeader_h
#define myHeader_h


#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <sstream>


using namespace std;


struct MyStruct
{
    float var1;
    float var2;
    float var3;
};

struct MyStruct2
{
    string var4;
    string var5;
    bool var6;
    MyStruct struct1;
};


int enterFunction(int maxNumber, int & currentNumber, structList);

#endif

Main Program

#include "myHeader.h"

int main()
{
    int currentNumber = 0, maxNumber = 0;

    MyStruct *structList;
    structList = new MyStuct[maxNumber];

    enterFunction(currentNumber, maxNumber, structList);
    return 0;
}

Functions file

#include "myHeader.h"

int enterFunction(int maxNumber, int & currentNumber, structList)
{
// DO MORE STUFF HERE
}

What error message are you getting? That should help you identify your issue. — rsjaffe, Dec 01 '18 at 22:34
You pass array of size 0 to the function (which can't resize it) ... — M.M, Dec 01 '18 at 23:49

Jens · Answer 1 · 2018-12-01T22:52:31.973

2

Instead of error-prone manual memory management you should use RAII to manage resources. C++ has std::vector for this and it makes life a lot easier. I guess that the function enterFunction should read a number of structs from a file or the keyboard, so the task would be to create a list of input values. You can pass a reference to the vector:

int enterFunction(int maxNumber, int & currentNumber, std::vector<MyStruct2>& structlist);

However, I would change the signature to return the vector with the input values. This makes the function easier to comprehend and reason about:

std::vector<MyStruct2> enterFunction(int maxNumber) {
    std::vector<MyStruct2> inputs;
    // let's assume that >> is overloaded for MyStruct2
    while (inputs.size() < maxNumber) {
        MyStruct2 x;
        cin >> x;
        inputs.push_back( std::move(x) );
    }

    return inputs;
}

Please note that this will not copy the vector as the return value because it uses named return value optimization.

edited Dec 01 '18 at 22:52

answered Dec 01 '18 at 22:44

Jens

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When done with a `std::vector` like this, both `maxNumber` and `currentNumber` are probably unnecessary. That's one of the reason `std::vector` rocks! – Eljay Dec 01 '18 at 22:48
I agree vector rocks, but for the user requirements of this program it has to be an array of structures (not my choice). :-) – RockRat Dec 01 '18 at 23:08
@RockRat You can pass a pointer to the vector's data to legacy code or C code. This is one of the use-cases for vector. – Jens Dec 02 '18 at 17:30

TonyK · Accepted Answer · 2018-12-01T23:10:41.290

In your header.h file, instead of

int enterFunction(int maxNumber, int & currentNumber, structList);

you need

int enterFunction(int maxNumber, int & currentNumber, MyStruct * structList);

And in your Functions file, likewise you need

int enterFunction(int maxNumber, int & currentNumber, MyStruct * structList)

And that's it!

By the way, it is dangerous to put using namespace std (or any using directive) in a header file like that. It might break other people's code if they have to include your header. Better to put it in your C file. (And even if nobody is ever going to include your header, it's a bad habit to get into.) See this stackoverflow question.

Compiled perfectly! That's what I needed. Thanks. – RockRat Dec 01 '18 at 23:14 — RockRat, Dec 01 '18 at 23:14

Can't figure out how to pass struct array to function

2 Answers2