Conversão NSString e std::string

1. Converta NSString em std::string

NSString *str = @"it is nsstring";  
std::string str = std::string([str UTF8String]);  

2.
Se não houver caracteres chineses para converter std::string em NSString, use:

string path = string("原图17.png");
NSString* pathString = [NSString stringWithCString:str.c_str() 
encoding:[NSString defaultCStringEncoding]];

Se houver caracteres chineses, use:

string path = string("原图17.png");
NSString* pathString = [NSString stringWithCString:path.c_str() 
encoding:NSUTF8StringEncoding];