[c++] What is the difference between _tmain() and main() in C++?

If I run my C++ application with the following main() method everything is OK:

int main(int argc, char *argv[]) 
{
   cout << "There are " << argc << " arguments:" << endl;

   // Loop through each argument and print its number and value
   for (int i=0; i<argc; i++)
      cout << i << " " << argv[i] << endl;

   return 0;
}

I get what I expect and my arguments are printed out.

However, if I use _tmain:

int _tmain(int argc, char *argv[]) 
{
   cout << "There are " << argc << " arguments:" << endl;

   // Loop through each argument and print its number and value
   for (int i=0; i<argc; i++)
      cout << i << " " << argv[i] << endl;

   return 0;
}

It just displays the first character of each argument.

What is the difference causing this?

the _T convention is used to indicate the program should use the character set defined for the application (Unicode, ASCII, MBCS, etc.). You can surround your strings with _T( ) to have them stored in the correct format.

 cout << _T( "There are " ) << argc << _T( " arguments:" ) << endl;

_tmain is a macro that gets redefined depending on whether or not you compile with Unicode or ASCII. It is a Microsoft extension and isn't guaranteed to work on any other compilers.

The correct declaration is

 int _tmain(int argc, _TCHAR *argv[]) 

If the macro UNICODE is defined, that expands to

int wmain(int argc, wchar_t *argv[])

Otherwise it expands to

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

Your definition goes for a bit of each, and (if you have UNICODE defined) will expand to

 int wmain(int argc, char *argv[])

which is just plain wrong.

std::cout works with ASCII characters. You need std::wcout if you are using wide characters.

try something like this

#include <iostream>
#include <tchar.h>

#if defined(UNICODE)
    #define _tcout std::wcout
#else
    #define _tcout std::cout
#endif

int _tmain(int argc, _TCHAR *argv[]) 
{
   _tcout << _T("There are ") << argc << _T(" arguments:") << std::endl;

   // Loop through each argument and print its number and value
   for (int i=0; i<argc; i++)
      _tcout << i << _T(" ") << argv[i] << std::endl;

   return 0;
}

Or you could just decide in advance whether to use wide or narrow characters. :-)

Updated 12 Nov 2013:

Changed the traditional "TCHAR" to "_TCHAR" which seems to be the latest fashion. Both work fine.

End Update

With a little effort of templatizing this, it wold work with any list of objects.

#include <iostream>
#include <string>
#include <vector>

char non_repeating_char(std::string str){
    while(str.size() >= 2){
        std::vector<size_t> rmlist; 
        for(size_t  i = 1;  i < str.size(); i++){        
            if(str[0] == str[i]) {
                rmlist.push_back(i);
            }      
        }          

        if(rmlist.size()){            
            size_t s = 0;  // Need for terator position adjustment   
            str.erase(str.begin() + 0);
            ++s;
            for (size_t j : rmlist){   
                str.erase(str.begin() + (j-s));                
                ++s;
            }
         continue;
        }
        return str[0];
   }
    if(str.size() == 1) return str[0];
    else return -1;
}

int main(int argc, char ** args)
{
    std::string test = "FabaccdbefafFG";
    test = args[1];
    char non_repeating = non_repeating_char(test);
    Std::cout << non_repeating << '\n';
}

Ok, the question seems to have been answered fairly well, the UNICODE overload should take a wide character array as its second parameter. So if the command line parameter is "Hello" that would probably end up as "H\0e\0l\0l\0o\0\0\0" and your program would only print the 'H' before it sees what it thinks is a null terminator.

So now you may wonder why it even compiles and links.

Well it compiles because you are allowed to define an overload to a function.

Linking is a slightly more complex issue. In C, there is no decorated symbol information so it just finds a function called main. The argc and argv are probably always there as call-stack parameters just in case even if your function is defined with that signature, even if your function happens to ignore them.

Even though C++ does have decorated symbols, it almost certainly uses C-linkage for main, rather than a clever linker that looks for each one in turn. So it found your wmain and put the parameters onto the call-stack in case it is the int wmain(int, wchar_t*[]) version.