在gcc和MSVC中以不同方式调用函数参数的析构函数

Question

While porting some C++ code from Microsoft Visual Studio to gcc, I ran into a weird bug, which I eventually boiled down to this:

#include <iostream>
using namespace std;

class Foo {
public:
    int data;
    Foo(int i) : data(i) 
    {
        cout << "Foo constructed with " << i << endl; 
    }
    Foo(const Foo& f) : data(f.data)
    {
        cout << "copy ctor " << endl;
    }
    Foo(const Foo&& f) : data(f.data)
    {
        cout << "move ctor" << endl;
    }
    ~Foo()
    {
        cout << "Foo destructed with " << data << endl;
    }
};

int Bar(Foo f)
{
    cout << "f.data = " << f.data << endl;
    return f.data * 2;
}

int main()
{
    Foo f1(10);
    Foo f2(Bar(std::move(f1)));
}

If I compile and run the above code with Microsoft Visual Studio 2015 Community, I get the following output:

Foo constructed with 10
move ctor
f.data = 10
Foo destructed with 10
Foo constructed with 20
Foo destructed with 20
Foo destructed with 10

However, if I compile and run the code with gcc 6.1.1 and --std=c++14, I get this output:

Foo constructed with 10
move ctor
f.data = 10
Foo constructed with 20
Foo destructed with 10
Foo destructed with 20
Foo destructed with 10

gcc calls the destructor of f , the argument to Bar() , after Bar() returns, while msvc calls the destructor (apparently) before it returns, or at least before the constructor of f2 . When is f supposed to be destructed, according to the C++ standard?

Answer 1

They are all right; it depends. It seems underspecified in the standard.

From [expr.call]/4 (this wording goes back to C++98);

The lifetime of a parameter ends when the function in which it is defined returns. The initialization and destruction of each parameter occurs within the context of the calling function.

And the CWG#1880 ;

WG decided to make it unspecified whether parameter objects are destroyed immediately following the call or at the end of the full-expression to which the call belongs.

Both the behaviour of g++ (and clang) and MSVC would be correct, implementations are free to pick one approach over the other.

That all said, if the code you have is dependent on this ordering, I would change it such that the ordering is more deterministic - as you have seen, it leads to subtle bugs.

---

A simplified example of this behaviour is;

#include <iostream>
struct Arg {
    Arg() {std::cout << 'c';}
    ~Arg() {std::cout << 'd';}
    Arg(Arg const&)  {std::cout << 'a';}
    Arg(Arg&&)  {std::cout << 'b';}
    Arg& operator=(Arg const&)  {std::cout << 'e'; return *this;}
    Arg& operator=(Arg&&)  {std::cout << 'f'; return *this;}
};
void func(Arg) {}
int main() {
    (func(Arg{}), std::cout << 'X');
    std::cout << std::endl;
}

Clang and g++ both produce cXd and MSVC produces cdX .