函数参数中const 引用 和 传值

#include <iostream>
using namespace std;

class Object
{
public:
Object(int i = 1) { n = i; cout << "Object::Object()" << endl; }
Object(const Object& a)
{
n = a.n;
cout << "Object::Object(const Object&)" << endl;
}
~Object() { cout << "Object::~Object()" << endl; }

void inc() { ++n; }
int val() const { return n; }

private:
int n;
};

void foo(Object a)
{
cout << "enter foo, before inc(): inner a = " << a.val() << endl;
//a.inc();
cout << "enter foo, after inc(): inner a = " << a.val() << endl;
}

int main()
{
Object a;

cout << "before call foo : outer a = " << a.val() << endl;
foo(a);
cout << "after call foo : outer a = " << a.val() << endl;

return 0;
}


可以进行测试便得知，传引用只需要调用构造函数一次，而传值需要2次。