Parsing arguments from main C++

Question

Possible Duplicate:
C String — Using Equality Operator == for comparing two strings for equality

Basic question here. I'm compiling this program in g++ and running it with a single -r argument, (./a.out -r) however it does not output the specified cout statement below. Is there anything wrong with this code?

#include <string>
using namespace std;

int main(int argc, char* argv[]) {

    if (argv[1] == "-r" ) {
        cout << "First arg is -r" << endl;
    }

    return 0;
}

score 6 · Accepted Answer · answered Nov 27 '12 at 10:06

6

You cannot compare string literals using ==, because what that does is compare pointers (i.e., memory addresses, which in this case are always going to be different).

Use strcmp or compare std::string objects instead:

if (strcmp(argv[1], "-r") == 0) { ... }

or

if (std::string(argv[1]) == "-r") { ... }

answered Nov 27 '12 at 10:06

Jon

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This is C++ though, not C. Can I still use strcmp? – thisiscrazy4 Nov 27 '12 at 10:10
@thisiscrazy4: That's because you tagged the question as C++. You can still use `strcmp` in C. – Jon Nov 27 '12 at 10:11
Yes it's C++, that's what I meant. – thisiscrazy4 Nov 27 '12 at 10:12
@thisiscrazy4: Ah, my mistake, sorry. Yes, both options are available in C++. – Jon Nov 27 '12 at 10:12
Ok that works fine in g++. However I get thread exceptions when compiling in Xcode. – thisiscrazy4 Nov 27 '12 at 10:22

score 1 · Answer 2 · answered Nov 27 '12 at 10:09

1

You can't compare string literals with ==. Use strcmp.

(Also, don't forget to check that argc is at least 2).

answered Nov 27 '12 at 10:09

Andrei Tita

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score 1 · Answer 3 · answered Nov 27 '12 at 10:26

One mistake in your code is you are comparing the address of the char array instead of comparing the values in the array. As people have said you can do it the C way using strcmp (although it's safer to use strncmp to avoid buffer overruns).

However, it's easier to use std::string in C++.

I would copy the args into a vector of strings.

#include <vector>
#include <string>

int main(int argc, char* argv[])
{
    vector<string> args(argv, argv+argc);
    ...

score 0 · Answer 4 · answered Nov 27 '12 at 10:12

Usually you would do something along these lines:

while (--argc && **++argv=='-')
{
   char *p;
    p=*argv+1;
    switch(tolower(*p))
    {
        case 'r':                                       
            break;

    }
}

basically, while there are some args left, check that there is a '-' char, then switch on the next char to see the modifier ('r', or 'a' etc)

Parsing arguments from main C++

4 Answers4