In which situations is the C++ copy constructor called?

Question

I know of the following situations in c++ where the copy constructor would be invoked:

1. when an existing object is assigned an object of it own class

Answer 1

Others have provided good answers, with explanations and references.

In addition, I have written a class to check the different type of instantations/assigments (C++11 ready), within an extensive test:

#include <iostream>
#include <utility>
#include <functional>


template<typename T , bool MESSAGES = true>
class instantation_profiler
{
private:
    static std::size_t _alive , _instanced , _destroyed ,
                       _ctor , _copy_ctor , _move_ctor ,
                       _copy_assign , _move_assign;


public:
    instantation_profiler()
    {
        _alive++;
        _instanced++;
        _ctor++;

        if( MESSAGES ) std::cout << ">> construction" << std::endl;
    }

    instantation_profiler( const instantation_profiler& )
    {
        _alive++;
        _instanced++;
        _copy_ctor++;

        if( MESSAGES ) std::cout << ">> copy construction" << std::endl;
    }

    instantation_profiler( instantation_profiler&& )
    {
        _alive++;
        _instanced++;
        _move_ctor++;

        if( MESSAGES ) std::cout << ">> move construction" << std::endl;
    }

    instantation_profiler& operator=( const instantation_profiler& )
    {
        _copy_assign++;

        if( MESSAGES ) std::cout << ">> copy assigment" << std::endl;
    }

    instantation_profiler& operator=( instantation_profiler&& )
    {
        _move_assign++;

        if( MESSAGES ) std::cout << ">> move assigment" << std::endl;
    }

    ~instantation_profiler()
    {
        _alive--;
        _destroyed++;

        if( MESSAGES ) std::cout << ">> destruction" << std::endl;
    }



    static std::size_t alive_instances()
    {
        return _alive;
    }

    static std::size_t instantations()
    {
        return _instanced;
    }

    static std::size_t destructions()
    {
        return _destroyed;
    }

    static std::size_t normal_constructions()
    {
        return _ctor;
    }

    static std::size_t move_constructions()
    {
        return _move_ctor;
    }

    static std::size_t copy_constructions()
    {
        return _copy_ctor;
    }

    static std::size_t move_assigments()
    {
        return _move_assign;
    }

    static std::size_t copy_assigments()
    {
        return _copy_assign;
    }


    static void print_info( std::ostream& out = std::cout )
    {
        out << "# Normal constructor calls: "  << normal_constructions() << std::endl
            << "# Copy constructor calls: "    << copy_constructions()   << std::endl
            << "# Move constructor calls: "    << move_constructions()   << std::endl
            << "# Copy assigment calls: "      << copy_assigments()      << std::endl
            << "# Move assigment calls: "      << move_assigments()      << std::endl
            << "# Destructor calls: "          << destructions()         << std::endl
            << "# "                                                      << std::endl
            << "# Total instantations: "       << instantations()        << std::endl
            << "# Total destructions: "        << destructions()         << std::endl
            << "# Current alive instances: "   << alive_instances()      << std::endl;
    }
};

template<typename T , bool MESSAGES>
std::size_t instantation_profiler<T,MESSAGES>::_alive       = 0;
template<typename T , bool MESSAGES>
std::size_t instantation_profiler<T,MESSAGES>::_instanced   = 0;
template<typename T , bool MESSAGES>
std::size_t instantation_profiler<T,MESSAGES>::_destroyed   = 0;
template<typename T , bool MESSAGES>
std::size_t instantation_profiler<T,MESSAGES>::_ctor        = 0;
template<typename T , bool MESSAGES>
std::size_t instantation_profiler<T,MESSAGES>::_copy_ctor   = 0;
template<typename T , bool MESSAGES>
std::size_t instantation_profiler<T,MESSAGES>::_move_ctor   = 0;
template<typename T , bool MESSAGES>
std::size_t instantation_profiler<T,MESSAGES>::_copy_assign = 0;
template<typename T , bool MESSAGES>
std::size_t instantation_profiler<T,MESSAGES>::_move_assign = 0;

Here is the test:

struct foo : public instantation_profiler<foo>
{
    int value;
};



//Me suena bastante que Boost tiene una biblioteca con una parida de este estilo...
struct scoped_call
{
private:
    std::function<void()> function; 

public:
    scoped_call( const std::function<void()>& f ) : function( f ) {}

    ~scoped_call()
    {
        function();
    }
};


foo f()
{
    scoped_call chapuza( [](){ std::cout << "Exiting f()..." << std::endl; } );

    std::cout << "I'm in f(), which returns a foo by value!" << std::endl;

    return foo();
}


void g1( foo )
{
    scoped_call chapuza( [](){ std::cout << "Exiting g1()..." << std::endl; } );

    std::cout << "I'm in g1(), which gets a foo by value!" << std::endl;
}

void g2( const foo& )
{
    scoped_call chapuza( [](){ std::cout << "Exiting g2()..." << std::endl; } );

    std::cout << "I'm in g2(), which gets a foo by const lvalue reference!" << std::endl;
}

void g3( foo&& )
{
    scoped_call chapuza( [](){ std::cout << "Exiting g3()..." << std::endl; } );

    std::cout << "I'm in g3(), which gets an rvalue foo reference!" << std::endl;
}

template<typename T>
void h( T&& afoo )
{
    scoped_call chapuza( [](){ std::cout << "Exiting h()..." << std::endl; } );

    std::cout << "I'm in h(), which sends a foo to g() through perfect forwarding!" << std::endl;

    g1( std::forward<T>( afoo ) );
}


int main()
{
    std::cout << std::endl << "Just before a declaration ( foo a; )"                << std::endl;                                        foo a;
    std::cout << std::endl << "Just before b declaration ( foo b; )"                << std::endl;                                        foo b;
    std::cout << std::endl << "Just before c declaration ( foo c; )"                << std::endl;                                        foo c;
    std::cout << std::endl << "Just before d declaration ( foo d( f() ); )"         << std::endl;                                        foo d( f() );

    std::cout << std::endl << "Just before a to b assigment ( b = a )"              << std::endl;                                        b = a;
    std::cout << std::endl << "Just before ctor call to b assigment ( b = foo() )"  << std::endl;                                        b = foo();
    std::cout << std::endl << "Just before f() call to b assigment ( b = f() )"     << std::endl;                                        b = f();



    std::cout << std::endl << "Just before g1( foo ) call with lvalue arg ( g1( a ) )"                         << std::endl;             g1( a );
    std::cout << std::endl << "Just before g1( foo ) call with rvalue arg ( g1( f() ) )"                       << std::endl;             g1( f() );
    std::cout << std::endl << "Just before g1( foo ) call with lvalue ==> rvalue arg ( g1( std::move( a ) ) )" << std::endl;             g1( std::move( a ) );

    std::cout << std::endl << "Just before g2( const foo& ) call with lvalue arg ( g2( b ) )"                          << std::endl;     g2( b );
    std::cout << std::endl << "Just before g2( const foo& ) call with rvalue arg ( g2( f() ) )"                        << std::endl;     g2( f() );
    std::cout << std::endl << "Just before g2( const foo& ) call with lvalue ==> rvalue arg ( g2( std::move( b ) ) )"  << std::endl;     g2( std::move( b ) );

  //std::cout << std::endl << "Just before g3( foo&& ) call with lvalue arg ( g3( c ) )"                         << std::endl;           g3( c );
    std::cout << std::endl << "Just before g3( foo&& ) call with rvalue arg ( g3( f() ) )"                       << std::endl;           g3( f() );
    std::cout << std::endl << "Just before g3( foo&& ) call with lvalue ==> rvalue arg ( g3( std::move( c ) ) )" << std::endl;           g3( std::move( c ) );



    std::cout << std::endl << "Just before h() call with lvalue arg ( h( d ) )"                         << std::endl;                    h( d );
    std::cout << std::endl << "Just before h() call with rvalue arg ( h( f() ) )"                       << std::endl;                    h( f() );
    std::cout << std::endl << "Just before h() call with lvalue ==> rvalue arg ( h( std::move( d ) ) )" << std::endl;                    h( std::move( d ) );

    foo::print_info( std::cout );
}

This is an abstract of the test compiled with GCC 4.8.2 with -O3 and -fno-elide-constructors flags:

Normal constructor calls: 10
Copy constructor calls: 2
Move constructor calls: 11
Copy assigment calls: 1
Move assigment calls: 2
Destructor calls: 19

Total instantations: 23
Total destructions: 19
Current alive instances: 4

Finally the same test with copy elision enabled:

Normal constructor calls: 10
Copy constructor calls: 2
Move constructor calls: 3
Copy assigment calls: 1
Move assigment calls: 2
Destructor calls: 11

Total instantations: 15
Total destructions: 11
Current alive instances: 4

Here is the complete code running at ideone.