How to Disable Copy Elision in C++ Compiler

c++copy-constructorinitialization

In c++98, the following program is expected to call the copy constructor.

#include <iostream>

using namespace std;
class A
{
  public:
    A() { cout << "default" ; }

    A(int i) { cout << "int" ; }


    A(const A& a) { cout << "copy"; }
};

int main ()
{
   A a1;
   A a2(0);
   A a3 = 0;

  return 0;
}

That is evident if you declare the copy constructor explicit in above case (the compiler errors out). But I don't I see the output of copy constructor when it is not declared as explicit. I guess that is because of copy elision. Is there any way to disable copy elision or does the standard mandates it?

Best Answer

Pre C++ 17

A a3 = 0;

will call copy constructor unless copy is elided. Pass -fno-elide-constructors flag

from C++17, copy elision is guaranteed. So you will not see copy constructor getting called.

Related Solutions

C++ – How to Disable Copy Constructor

You can make the copy constructor private and provide no implementation:

private:
    SymbolIndexer(const SymbolIndexer&);

Or in C++11, explicitly forbid it:

SymbolIndexer(const SymbolIndexer&) = delete;

C++ Compiler Directives – How to Ensure Copy Elision

No.

But you can write an equivalent, although completely unreadable, code:

BigObj f()
{
    BigObj x(g());
    x.someMethod();
    return x;
}

//...
BigObj z = f();
//...

is translated (with copy elision) to:

void f(BigObj* obj)
{
    new(obj) BigObj(g());
    obj->someMethod();
}

//...
char z[sizeof(BigObj)];
f((BigObj*)&z[0]);
//...
((BigObj*)&z[0])->~BigObj();

But seriously, just write your code in such a way that the compiler can elide the copy. I.e. return only one object without branching:

BigObj f()
{
    BigObj x, y;
    // use x and y
    if(condition)
        return x;
    else
        return y;
    // cannot be elided
}


BigObj f()
{
    if(condition)
    {
        BigObj x;
        return x;
    }
    else
    {
        BigObj y;
        return y;
    }
    // can be elided
}

Best Answer

Related Solutions

C++ – How to Disable Copy Constructor

C++ Compiler Directives – How to Ensure Copy Elision

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