C++临时对象的生命期

class Test{
public:
    Test(int a):m_int(a){
        printf("this is Test(%d) ctor\n", m_int);
    }
    ~Test(){
        printf("this is Test(%d) dtor\n", m_int);
    }
    const Test & testfun() const{
        printf("this is %d testfun\n", m_int);
        return *this;
    }
private:
    int m_int;
};

struct S {
    int mi;
const Test& mt;
    ~S(){
        printf("this is S dtor\n");
    }
};

Whenever a reference is bound to a temporary or to a subobject thereof, the lifetime of the temporary is extended to match the lifetime of the reference, with the following exceptions:

当临时对象绑定到引用上时，临时对象的生命期也会跟随引用的生命期而得到延长，但是也有一些特殊情况：

a temporary bound to a return value of a function in a return statement is not extended: it is destroyed immediately at the end of the return expression. Such function always returns a dangling reference.

对于返回引用的函数，如果函数中的return语句返回的是一个临时对象，则该临时对象在return语句的最后就会被销毁。这种情况下，该函数返回的引用实际上就是个“空悬引用”（dangling reference）。实际上即使不是临时对象，只要是返回函数内部对象的引用，在return之后，该引用就是空悬引用了：

const Test & fun(){
    return Test();
}

int main(int argc, char **argv){
    const Test &aa = fun();
    printf("over\n");
    aa.testfun();
}

上面的代码，在编译时就会报警：warning: returning reference to temporary [-Wreturn-local-addr]  return Test(3);

代码结果如下：

this is Test() ctor
this is Test() dtor
over
Segmentation fault (core dumped)

实际上，最后的段错误有可能发生，也有可能不发生，重要的是，const Test &aa = fun();之后，aa已经是个空悬引用了。

a temporary bound to a reference member in a constructor initializer list persists only until the constructor exits, not as long as the object exists. (note: such initialization is ill-formed as of DR 1696).  (until C++14)

在initializer list形式的构造函数中，如果临时对象绑定到一个对象A的引用成员中，则该临时对象的生命期在构造函数结束后就会结束，并不跟随对象A的生命期而得到延长。但是这一条规则在C++14之后就不再使用了。下面的代码验证了这一点：

    S ss {, Test() };
    printf("over\n");
    ss.mt.testfun();

代码的结果如下：

this is Test() ctor
over
this is  testfun
this is Test() dtor
this is S dtor

可见，S中的引用成员mt绑定到了一个临时对象，该临时对象的生命期跟随ss而得到了延长。

a temporary bound to a reference parameter in a function call exists until the end of the full expression containing that function call: if the function returns a reference, which outlives the full expression, it becomes a dangling reference.

如果函数参数为引用，当临时对象绑定到该参数上时，其生命期一直持续到包含该函数调用的表达式的结束，如果该函数又返回了该引用，则该引用成为空悬引用：

const Test & fun3(const Test &a){
    a.testfun();
    return a;
}

int main(int argc, char **argv){
    const Test &a = fun3(Test()).testfun();
    printf("over\n");
    a.testfun();
}

代价结果如下：

this is Test() ctor
this is  testfun
this is  testfun
this is Test() dtor
over
this is  testfun

尽管最后的testfun打印出了结果，但是这是未定义行为。

a temporary bound to a reference in the initializer used in a new-expression exists until the end of the full expression containing that new-expression, not as long as the initialized object. If the initialized object outlives the full expression, its reference member becomes a dangling reference.

如果临时对象绑定到了一个在new表达式中初始化的引用，则该临时对象的生命期一直持续到包含new的表达式的结束，而不是跟随new出来的对象的生命期而得到延长，此时，该对象中的引用成员在表达式结束之后，就成了空悬引用：

    S *p = new S{, Test() };
    printf("p->mi is %d\n", p->mi);
    p->mt.testfun();

代码结果如下：

this is Test() ctor
this is Test() dtor
over
p->mi is
this is  testfun

尽管最后的testfun打印出了结果，但是这是未定义行为。

In general, the lifetime of a temporary cannot be further extended by "passing it on": a second reference, initialized from the reference to which the temporary was bound, does not affect its lifetime.

一般而言，临时对象的生命期并不能随着引用的传递而得到延长：如果临时对象绑定到了引用A，而使用引用A又初始化了引用B，这种情况下，引用B对临时对象的生命周期没有影响。

https://en.cppreference.com/w/cpp/language/reference_initialization