What's the correct way to add a character array to a constant character array in C++?

Question

What's the correct way to add a character array to a constant character array in C++?

#include <iostream>
using namespace std;

int main(int argc, char** argv) {
    int pathSize = 0;
    char* pathEnd = &argv[0][0];
    while(argv[0][pathSize] != '\0') {
        if(argv[0][pathSize++] == '/')
            pathEnd = &argv[0][0] + pathSize;
    }
    pathSize = pathEnd - &argv[0][0];
    char *path = new char[pathSize];
    for(int i = 0; i < pathSize; i++)
        path[i] = argv[0][i];
    cout << "Documents Path: " << path + "docs/" << endl; // Line Of Interest
    delete[] path;
    return 0;
}

This code outputs: 
Documents Path: �\

Using 'path' instead of '*path' will give me the compile error: 
invalid operands of types ‘char*’ and ‘const char [6]’ to binary ‘operator+’

Answer 1

May I suggest using C++ to begin with, and (Boost) Filesystem for maximum benefits:

#include <iostream>
#include <boost/filesystem.hpp>
using namespace std;
using namespace boost::filesystem;

int main(int argc, const char *argv[])
{
    const std::vector<std::string> args { argv, argv+argc };

    path program(args.front());
    program = canonical(program);
    std::cout << (program.parent_path() / "docs").native();
}

This will use the platform's path separator, know how to translate 'funny' paths (e.g. containing ..\..\, or UNC paths).

Answer 2

Something like this should do it (totally untested):

const char* end = strrchr(argv[0], '/');
std::string docpath = end ? std::string(argv[0], end) : std::string(".");
docpath += '/docs/';

Answer 3

Your way:

#include <iostream>
using namespace std;

int main(int argc, char** argv) {
    int pathSize = 0;

    char* pathEnd = &argv[0][0];
    while(argv[0][pathSize] != '\0') {
        if(argv[0][pathSize++] == '/')
            pathEnd = &argv[0][0] + pathSize;
    }

    pathSize = pathEnd - &argv[0][0];
    char *path = new char[pathSize + 5]; //make room for "docs/"
    for(int i = 0; i < pathSize; i++) 
        path[i] = argv[0][i];

    char append[] = "docs/";
    for(int i = 0; i < 5; i++) 
        path[pathSize+i] = append[i];

    cout << "Documents Path: " << path << endl;
    function_expecting_charptr(path);

    delete[] path;
    return 0;
}

Sane C way:

#include <iostream>
using namespace std;

int main(int argc, char** argv) {

    char* pathEnd = strrchr(argv[0], '/');
    if (pathEnd == NULL)
        pathEnd = argv[0];
    int pathSize = (pathEnd-argv[0]) + 5; //room for "docs/"

    char *path = new char[pathSize];
    if (pathSize)
        strncpy(path, argv[0], pathSize+1);
    strcat(path, "docs/");

    cout << "Documents Path: " << path << endl;
    function_expecting_charptr(path);

    delete[] path;
    return 0;
}

C++ way:

#include <iostream>
#include <string>

int main(int argc, char** argv) {
    std::string path = argv[0];

    size_t sep = path.find('/');
    if (sep != std::string::npos)
        path.erase(sep+1);
    else
        path.clear();

    path += "docs/";
    std::cout << "Documents Path: " << path << endl;
    function_expecting_charptr(path.c_str());

    return 0;
}

Note that argv[0] holds an implementation defined value, and especially in *nix environments isn't guaranteed to hold anything useful. The first parameter passed to the program is found in argv[1].

Answer 4

I combined some of your guys' ideas into this compact code:

#include <iostream>
#include <cstring>
using namespace std;

int main(int argc, char** argv) {
    const string path_this = argv[0];
    const string path = path_this.substr(0, strrchr(argv[0], '/') - argv[0] +1);
    const string path_docs = path + "docs/";
    cout << "Documents Path: " << path_docs << endl;
    return 0;
}

To get the character array from this I can then run 'path_docs.c_str()'.

Credits: @MooingDuck, @MarkB, Google.