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

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

Check out QDir for that:

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

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;
}

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.

If you want to do this with Qt, you can use QFileInfo constructor:

QFileInfo fi( QDir("/tmp"), "file" );
QString path = fi.absoluteFilePath();

With C++11 and Qt you can do this:

QString join(const QString& v) {
    return v;
}

template<typename... Args>
QString join(const QString& first, Args... args) {
    return QDir(first).filePath(join(args...));
}

Usage:

QString path = join("/tmp", "dir", "file"); // /tmp/dir/file

In Qt, just use / in code when using Qt API (QFile, QFileInfo). It will do the right thing on all platforms. If you have to pass a path to a non-Qt function, or want to format it for displaying it to the user, use QDir:toNativeSeparators() e.g.:

QDir::toNativeSeparators( path );

It will replace / by the native equivalent (i.e. \ on Windows). The other direction is done via QDir::fromNativeSeparators().