How to build a full path string (safely) from separate strings?

At least in Unix / Linux, it's always safe to join parts of a path by /, even if some parts of the path already end in /, i.e. root/path is equivalent to root//path.

In this case, all you really need is to join things on /. That said, I agree with other answers that boost::filesystem is a good choice if it is available to you because it supports multiple platforms.

Check out QDir for that:

QString path = QDir(dirPath).filePath(fileName);

Only as part of Boost.Filesystem library. Here is an example:

#include <iostream>
#include <boost/filesystem.hpp>

namespace fs = boost::filesystem;

int main ()
{
    fs::path dir ("/tmp");
    fs::path file ("foo.txt");
    fs::path full_path = dir / file;
    std::cout << full_path << std::endl;
    return 0;
}

Here is an example of compiling and running (platform specific):

$ g++ ./test.cpp -o test -lboost_filesystem -lboost_system
$ ./test 
/tmp/foo.txt

Similar to @user405725's answer (but not using boost), and mentioned by @ildjarn in a comment, this functionality is available as part of std::filesystem. The following code compiles using Homebrew GCC 9.2.0_1 and using the flag --std=c++17:

#include <iostream>
#include <filesystem>
namespace fs = std::filesystem;

int main() 
{
    fs::path dir ("/tmp");
    fs::path file ("foo.txt");
    fs::path full_path = dir / file;
    std::cout << full_path << std::endl;
    return 0;
}