我对C++的返回值机制感到困惑，我写了以下代码来证明我的观点，代码的结果（有“？”在它之后，并且输出是粗体）让我感到困惑，谁能解释它为什么输出是这样，还是仅仅因为我的编译器供应商（MS Visual C++）为我优化？
#include <iostream>
class WInt
{
public:
WInt( int a ) : a(a){ std::cout << a << " " << "A constructor" << std::endl; }
WInt( const WInt& a )
{
std::cout << "copy constructor run" << std::endl;
this->a = a.a;
}
~WInt(){ std::cout << "WInt destructor" << std::endl; }
WInt& operator=( const WInt& v )
{
std::cout << "assignment operator" << std::endl;
this->a = v.a;
return *this;
}
friend const WInt operator+( const WInt& v1, const WInt& v2 )
{
return WInt( v1.a + v2.a );
}
private:
int a;
};
int main( int argc, char* argv[] )
{
std::cout << "-----------" << std::endl;
WInt a(1); // run constructor
WInt b(2); // run constructor
std::cout << "-----------" << std::endl;
WInt c = a + b; // ???????????????????
std::cout << "-----------" << std::endl;
WInt d( a + b ); // ???????????????????
std::cout << "-----------" << std::endl;
c = a + b + c; // run the +, +, =, ~, ~
std::cout << "-----------" << std::endl;
WInt e = c; // run copy constructor
std::cout << "-----------" << std::endl;
return 0;
}
输出是：
-----------
1 A constructor
2 A constructor
-----------
**3 A constructor**
-----------
**3 A constructor**
-----------
3 A constructor
6 A constructor
assignment operator
WInt destructor
WInt destructor
-----------
copy constructor run
-----------
WInt destructor
WInt destructor
WInt destructor
WInt destructor
WInt destructor