Re: [問題] const reference
※ 引述《Arton0306 (Ar藤)》之銘言：
: 因為程式碼比較短也直接貼在下方(http://codepad.org/o7M1WcFG)
: 程式碼輸出為
: 500
: 0
: 0
: 500
: 500
: 500
: 123
: 123
: 123
: 請問為什麼 int const & x 以 500 初始化後 此值會被洗掉
: 而 static int const &k = 123 這個值不會被洗掉
: 這背後的機制是什麼呢？
: ===========================================================
: #include <iostream>
: #include <cstdio>
: using namespace std;
: class foo
: {
: public:
: int const & x;
: foo():x(500)
: {
: }
: };
: void test2()
: {
: static int const &k = 123;
: cout<<k<<endl;
: }
: int main()
: {
: foo a;
: cout<<a.x<<endl;
: cout<<a.x<<endl;
: cout<<a.x<<endl;
: const int &cir = 500;
: cout<<cir<<endl;
: cout<<cir<<endl;
: cout<<cir<<endl;
: test2();
: test2();
: test2();
: return 0;
: }
Background: const reference可以用來延長暫時物件的壽命：
int f()
{
return 42;
}
{
const int& x = f(); // The temporary int is bound to x.
assert(x == 42);
} // The temporary int returned from f() is actually destroyed here!
----
但是有一些例外，參見C++ Standard §12.2.5 [class.temporary]：
The temporary to which the reference is bound or the temporary that is the
complete object to a subobject of which the temporary is bound persists for
the lifetime of the reference except as specified below. A temporary bound to
a reference member in a constructor's ctor-initializer (12.6.2) persists
until the constructor exits. A temporary bound to a reference parameter in a
function call (5.2.2) persists until the completion of the full expression
containing the call.
class foo
{
public:
foo()
: x(42)
{
} // The temporary int 42 is destroyed here!!!
const int& x;
};
{
foo f;
// f.x is a dangling reference here now.
}
----
void f()
{
static const int& x = 42;
}
這裡因為x的lifetime其實是從第一次呼叫f到程式結束，
所以這個暫時物件也會被延長到程式結束，可以安心使用
--
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