CSTDIO -> Ładowanie całej zawartości pliku do stringa.

fseek( file, 0, SEEK_END );
long size = ftell( file );
fseek( file, 0, SEEK_SET ); //lub rewind(file);
char * data = new char[ size ];
fgets( data, size, file ); //lub fread

#include <cstdio>
#include <string>

int main()
{
    char charFromFile = '\0';
    FILE * file = 0;
    std::string fileData;
   
    file = fopen( "wtf.txt", "r" );
    if( !file )
         return 1;
   
    while( charFromFile != EOF )
    {
        charFromFile = fgetc( file );
        fileData += charFromFile;
    }
    fclose( file );
   
    printf( "File data:\n%s", fileData.c_str() );
    return 0;
}