debug assertion error - c++

Question

I've been scratching my head for quite some time now, this code worked fine when I first used cmd to go inside the project\debug folder then run the program there. Then I added the if(in) and else part then it started giving me "debug assertion failed" errors mbstowcs.c
Expression s != NULL
It just doesn't make any sense to me..
I used this command in cmd: prog.exe test.txt nuther.txt
Both files exists inside the debug folder and the main project folder..
Any ideas?
int main(int argc, char **argv)
{
parse_opts(argc, argv); //parse the arguments
return 0;
}
void parse_opts(int argc, char **argv)
{
string compl_out;
if( argc > 1 )
{
for( int i = 1; i < argc; i++ )
{
if( argv[i][0] = '>' )
{
ofstream out_file(argv[i+1]);
out_file << compl_out;
out_file.close();
break;
}
ifstream in(argv[i]);
string buff;
if(in)
{
while(getline( in, buff ))
cout << buff << endl;
compl_out.append(buff);
}
else
{
cout << "Can't open file: " << argv[i]
<< ", file doesn't exist or is locked in use. " << endl;
}
}
}
else
{
usage();
}
}

Answer 1

First impressions:
if( argv[i][0] = '>' )
should be:
if( argv[i][0] == '>' )
You are assigning instead of comparing.
I think you also might have intended the compl_out.append to be inside the while loop? As it is it won't append anying to that buffer:
while(getline( in, buff ))
{
cout << "buf" << buff << endl;
compl_out.append(buff);
}