Function to concat two char* in C++

Question

First off, I'm a C# programmer, so my working knowledge of C++ is fairly limited. I took it back in college, but haven't touched it in 10 years, so please forgive me if this is relatively simple stuff.

I'm attempting to make a DLL that I can use in C# that implements the libwpd library.

I've managed to create a DLL that exports 2 functions that I can access via P/Invoke. The first returns a constant integer (generated by visual studio as a sample), the 2nd a string.

If I return a constant string from the function, it passes successfully to C# and I can read it on the other end, so I know the data is being passed back.

The problem I'm running into is with libwpd. I've had to modify their TextDocumentGenerator.cpp file to add the information to a char* instead of using the printf that they use so I can access it later.

I've added a variable definition to the public section of the header file so I can read it from the calling code.

Now, I'm trying to write a function that allows me to add the char* given by libwpd to the external char*.

I've come up with this:

char* addString(const char* addThis, char* toThis)
{
char* copier  = (char*)malloc(strlen(toThis) + 1 + 1);
strcpy(copier, toThis);
strcpy(copier, "1");

toThis = (char*)malloc(strlen(copier) + 1);
strcpy(toThis, copier);

return copier;
} 

But when I pass the information back, I get a blank string.

I call the function by calling totalFile = addString("\n", totalFile);

(I realize it should only technically add "1" to the string repeatedly, but it's not doing even that)

If i change the strcpy to strcat for the copier lines, it locks up.

I don't know how to create a program in C++ so I can even step through the functions to see what's happening.

Any assistance would be appreciated.

Answer 1

Are you aware of existence of std::string? It's a class that handles strings in C++; char * is legacy from C.

std::string provides + operator, that does what you want.

Answer 2

You need to allocate enough space for the return string, copy the inital string into the destination buffer, and then call strcat to append the extra info.

For example:

char* addString(const char* addThis, const char* toThis)
{
    char* destination = (char*)malloc( strlen( addThis ) + strlen( toThis ) + 1 );
    strcpy( destination, toThis );
    strcat( destination, addThis );
    return destination;
} 

Don't forget you'll need to call free( destination ) at some point after calling this function.

EDIT: Of course, this is just me fixing the function proposed in your question. Really, it's not a great idea to return a pointer from a function and count on the caller to free it. Since you're using C++ rather than C, you'd likely be much better off using some C++ constructs, such as std::string or at the very least, wrapping the char* in a shared_ptr or similar.

If you're going to be writing C++, I'd recommend buying yourself a good book on the subject, it really is far too easy to shoot yourself in the foot if you don't know what you're doing.

Answer 3

Since you are replacing a printf() call, sprintf() would probably be the easiest way to create your string.

Answer 4

First off, Griwes is correct. You have no business doing what you are doing and marking this question as C++. The simple reason being, this is not how we work with strings in C++.

As you are a C# programmer you are well aware of the String class. Well, std::string is similar. It takes the leg-work out of dealing with strings so that you do not have to go down the painful route that you're going down.

To concatenate two strings together in C++, it's as simple as:

std::string a = "I slept with ";
std::string b = "a heavy heart.";
std::string c = a + b;

Voila.