Will (N)RVO be applied with my function in this situation?

Question

I have the following code: (ok, in reality it's much more complicated, but I simplified it to make it easier to understand. so please disregard the things that seems stupid. I can't change them in my real situation)

#include <string>

using std::string;

ReportManager g_report_generator;

struct ReportManager
{
    // I know, using c_str in this case is stupid. 
    // but just assume that it has to be this way
    string GenerateReport() { string report("test"); return report.c_str(); }
}

string DoIt(bool remove_all)
{
    if(g_report_generator.isEmpty())
        return string();

    string val = g_report_generator.GenerateReport();

    if(remove_all)
        g_report_generator.clear();

    return val;
}

void main()
{
    string s = DoIt(true);
}

Will (N)RVO be applied with my functions? I did a bit of research, and it would seem like it, but I'm not really convinced and I'd like a second opinion (or more).

I'm using Visual Studio 2017.

Answer 1

To solve your problem I rewrote it.

#include <string>


struct string : std::string {
    using std::string::string;

    string(string&& s) {
        exit(-1);
    }
    string(string const&) {
        exit(-2);
    }

    string() {}
};

struct ReportManager
{
    // I know, using c_str in this case is stupid. 
    // but just assume that it has to be this way
    string GenerateReport()
    {
        string report("test");
        return report.c_str();
    }
    bool isEmpty() const { return true; }
    void clear() const {}
};

ReportManager g_report_generator;

string DoIt(bool remove_all)
{
    if(g_report_generator.isEmpty())
        return string();

    string val = g_report_generator.GenerateReport();

    if(remove_all)
        g_report_generator.clear();

    return val;
}

int main()
{
    string s = DoIt(true);
}

The trick with this rewriting is that elision permits skipping copy/move ctors. So every time we actually copy an object (even if inlined), we'll insert an exit clause; only by elision can we avoid it.

GenerateReport has no (N)RVO or any kind of elision, other than possibly under as-if. I doubt a compiler will be able to prove that, especially if the string is non-static and large enough to require heap storage.

For DoIt both NRVO and RVO is possible. Elision is legal there, even with side effects.

MSVC fails -- notice calls to ??0string@@QAE@$QAU0@@Z , which is the move constructor of my local string class.

When I force the possible RVO case to run by saying it is empty , you'll see that the compiler also fails to RVO optimize here; there is an exit(-1) inlined into the disassembly.

Clang manages to RVO the return string(); but not NRVO the return val; .

By far the easiest fix is:

string DoIt(bool remove_all)
{
    if(g_report_generator.isEmpty())
        return string();

    return [&]{   
      string val = g_report_generator.GenerateReport();

      if(remove_all)
        g_report_generator.clear();

      return val;
    }();
}

which has double RVO, and a lambda that does simple NRVO. Zero structural changes to your code, and functions which C++98 compilers can elide return values on (well, they don't support lambda, but you get the idea).

Answer 2

I do not think (N)RVO is possible in either functions. GenerateReport has to construct a string from character array, there is nothing left for NRVO. DoIt returns two different values through it control path, which makes it impossible to perform NRVO as well.