Thread: Returning const char*/ std::string.c_str()

Hi,

I have a function in a program that's something like the following. When I try to output what it returns, however, it doesn't print anything on the screen. The following isn't exactly what I'm using, but it gets the point across.

Code:

const char* getcolor(bool top)
{
    stringstream color_convert;
    string string1;

    if (top == 1)
        color_convert << 160;
    else
        color_convert << 200;

    string1 = color_convert.str();
    return string1.c_str();
}

Alternately, if I replace the contents of the function with:

Code:

const char* c_string1 = "160";
return c_string1;

then it will return something that I can print out. Does anyone know why this is? I'm guessing it's something to do with pointers- like in the first function, the pointer to the string is empty after it's returned or something? It's weird though because both functions use pointers to stuff. I don't see why one should be empty and the other not.

Thanks.

Is there a reason you don't just return a string?

I ask because I find a lot of people convert to C style strings because they are unaware that the C++ string works in their situation as well.

Originally Posted by Daved

Is there a reason you don't just return a string?

I ask because I find a lot of people convert to C style strings because they are unaware that the C++ string works in their situation as well.

I'm returning a c string because basically, I'm writing a mod for the game Half-Life. The only way you can print messages to the screen in Half-Life is via c_strings (Half-Life is written in C++ but it's based on the Quake 1 engine which is C). You have to use functions that are part of the Half-Life code to do this.

I was wondering something though. The reason I ask this question is because my mod basically uses a class that outputs a load of stuff to a stringstream object. I also have some interface classes as part of my project. One of these just outputs the stringstream stuff to cout. Another outputs stuff via Half-Life. This one has some make_cstring_msg() member functions that convert the stuff in the stringstream to c_strings. Would I be better off just returning strings and then converting these to c_strings in the Half-Life code itself? By "Half-Life code itself" I mean basically you have to have this file called dll.cpp, and that's what interfaces with Half-Life. Hope that makes sense.

Thanks.

Originally Posted by Elysia

You cannot even return a string in C (unless it's a char*, but then we have to be talking about malloc and who is responsible for freeing it). So, are you sure you need to RETURN a string, not copy it into some buffer?

Well to be honest, I don't know. Right now I do it via something like the following, which is one of the "make c string" functions I mentioned.

Code:

const char *get_th_choice_cstr()    { return choice_strings[th_choice-1].c_str(); }

Presumably this is the wrong way to do it, and I'd be better off by having get_th_choice_cstr() declared as having an argument that's a pointer to a c string, right? So...

Code:

void get_th_choice_cstr(char *cstring1)    { strcpy(cstring1, choice_strings[th_choice-1].c_str()); }

Would something like that make sense?

Thanks for your help.

Originally Posted by anon

The first one shouldn't be wrong either, because now choice_strings is not a local variable.

Yeah, I've been using that for a while and it seems to work. Still, is it better to do it the second way? Or doesn't it really matter? I have occasionally noticed the text screwing up in the game- bits of old messages being added onto the end of new ones etc. But I dunno if that's to do with this or just the game itself.