Append Const Char to Char* C++

Question

I know this sounds like a beginner question but I have never worked with char* in c++. I always just used strings. But there is a bug with Ubuntu 13.10 and the NVidia OpenGL drivers so I have to use char*. Here is the bug.

I have a class that takes a const char* folderName in the constructor and then creates compiles all the .glsl files in the folder. I have tried strcpy and strcat and they both seem so override.

glProgram *p = new glProgram("phong\n");
glProgram :: glProgram(const char* folderName)
{
    cout << folderName << endl;
    char* name = strdup(folderName);
    cout << name << endl;
    name = strcat(name,"/vertex.glsl");     
    cout << name << endl;
}

But the output is not as expected.

phong
phong
/vertex.glsl

How can I get the output to be phong/vertex.glsl ?

Answer 1

The code is invalid because you overwrite memory that does not belong to the dynamically allocated array. You should yourself allocate a memory region large enough to contain the concatenated string.

glProgram *p = new glProgram("phong\n");

glProgram :: glProgram(const char *folderName)
{
    const char *vertex = "/vertex.glsl";

    size_t n = std::strlen( folderName );
    if ( n != 0 && folderName[n - 1] == '\n' ) --n;

    char *name = new char[n + std::strlen( vertex ) + 1];

    std::strncpy( name, folderName, n );
    name[n] = '\0';
    std::strcat( name, vertex ); 
}

Also I do not see any sense that the argument includes the new line character. Why may not the argument look as "phong" ?