Question

I’m wondering whether this bit of code is exhibiting the correct C++ behaviour?

class Foo
{
public:
    Foo(std::string name) : m_name(name) {}

    Foo(const Foo& other) { 
        std::cout << "in copy constructor:" << other.GetName() << std::endl;
        m_name = other.GetName();
    }

    std::string GetName() const { return m_name; }
    void SetName(std::string name) { m_name = name; }

private:
    std::string m_name;
};

Foo CreateFoo(std::string name)
{
    Foo result(name);
    return result;
}

void ChangeName(Foo& foo)
{
    foo.SetName("foofoo");
}

int _tmain(int argc, _TCHAR* argv[])
{
    Foo fooA("alan");
    std::cout << "fooA name: " << fooA.GetName() << std::endl;
    bool b = true;
    ChangeName(b ? fooA : CreateFoo("fooB"));
    std::cout << "fooA name: " << fooA.GetName() << std::endl;
    return 0;
}

When built in VS2008 the output is:

fooA name: alan
fooA name: foofoo

But when the same code is built in VS2010 it becomes:

fooA name: alan
in copy constructor: alan
fooA name: alan

A copy constructor is being invoked on ‘alan’ and, despite being passed by reference (or not as the case may be), fooA is unchanged by the called to ChangeName.

Has the C++ standard changed, has Microsoft fixed incorrect behaviour or have they introduced a bug?

Incidentally, why is the copy constructor being called?

Answer 1

A fuller answer:

5.16/4&5:

“4 If the second and third operands are lvalues and have the same type the result is of that type and is an lvalue.

5 Otherwise the result is an rvalue….”

In other words, “bool ? lvalue:rvalue” results in a temporary.

That would be the end of it, however you pass this into a function that, according to C++, MUST receive an lvalue as parameter. Since you pass it an rvalue you actually have code that is not C++. MSVC++ accepts it because it’s stupid and uses a bunch of extensions it doesn’t tell you about unless you turn it into a pendant. Since what you have is not standard C++ to begin with, and MS is just allowing it by extension, nothing can really be said about what is “correct” regarding it anymore.