Problems with the comparison of two strings using strcmp

Question

I want to compare 2 string but when I do a strcmp function, it tells me that:

'strcmp' : cannot convert parameter 1 from 'std::string'

How can I fix this?

Here is my code :

int verif_file(void)
{
    string ligne;
    string ligne_or;

    ifstream verif("rasphone");
    ifstream original("rasphone.pbk");
    while (strcmp(ligne, "[SynCommunity]") != 0 &&
        (getline(verif, ligne) && getline(original, ligne_or)));    
    while (getline(verif, ligne) && getline(original, ligne_or))
    {
        if (strcmp(ligne, ligne_or) != 0)
            return (-1);
    }

    return (0);
}

Answer 1

Your compiler gives you an error because strcmp is a C-style function that expects arguments of type const char* and there is no implicit conversion from std::string to const char*.

And although you might retrieve a pointer of this type using std::string's c_str() method, since you are working with std::string objects, you should use the operator == instead:

if (ligne == ligne_or) ...

or comparison with const char*:

if (ligne == "[Syn****]") ...

Answer 2

Just use std::string's operator==:

if (ligne == "[SynCommunity]") ...

if (ligne == ligne_or) ...

Answer 3

Change

if (strcmp(ligne, ligne_or) != 0)

to

if (ligne != ligne_or)

Answer 4

If you want to use strcmp, then try

if (strcmp(ligne.c_str(), ligne_or.c_str()) != 0)
   ...

Answer 5

I like the boost algorithm library.

#include <boost/algorithm/string.hpp>

std::string s1("This is string 1");
std::string s2("this is string 2");

namespace balg = boost::algorithm;

// comparing them with equals
if( balg::equals( s1, s2 ) ) 
     std::cout << "equal" << std::endl;
else
     std::cout << "not equal" << std::endl;

// case insensitive  version
if( balg::iequals( s1, s2 ) ) 
     std::cout << "case insensitive equal" << std::endl;
else
     std::cout << "not equal" << std::endl;