其他人已经提供了很好的答案,包括解释和参考资料。
此外,我编写了一个类来检查不同类型的实例化/赋值(C++11准备就绪),并进行了广泛的测试:
#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;
这里是测试:
struct foo : public instantation_profiler<foo>
{
int value;
};
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 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 );
}
这是使用
GCC 4.8.2
编译,带有
-O3
和
-fno-elide-constructors
标志的测试摘要:
普通构造函数调用:10
复制构造函数调用:2
移动构造函数调用:11
复制赋值函数调用:1
移动赋值函数调用:2
析构函数调用:19
总实例化数:23
总销毁数:19
当前存活实例数:4
最后,启用复制省略功能的相同测试结果:
普通构造函数调用:10
复制构造函数调用:2
移动构造函数调用:3
复制赋值函数调用:1
移动赋值函数调用:2
析构函数调用:11
总实例化数:15
总销毁数:11
当前存活实例数:4
这里是在ideone上运行的完整代码。
A=B;
会调用复制赋值运算符。 - BWGA = new MyClass();
无法编译。 - Andy ProwlA
之后才能进行赋值操作。例如:A a; ... a=b;
。如果在声明时进行赋值操作,则A a=b
等同于A a(b)
。 - barak manos