[C++11] decltype的使用

[wgxabc]

int i =4;
decltype(++i) var9 = i; //var9:int& ++i返回i的左值
decltype(i++) var10; //var10:int i++返回右值

问题：
为什么上面的++i和i++返回的不一样呢？

[pengzhixi]

The operand of prefix ++ is modified by adding 1, or set to true if it is bool (this use is deprecated). The operand shall be a modifiable lvalue. The type of the operand shall be an arithmetic type or a pointer to a completely-defined object type. The result is the updated operand; it is an lvalue. The value of a postfix ++ expression is the value of its operand. [Note: the value obtained is a copy of the original value —end note ] The operand shall be a modifiable lvalue. The type of the operand shall be an arithmetic type or a pointer to a complete object type. The value of the operand object is modified by adding 1 to it, unless the object is of type bool, in which case it is set to true. [Note: this use is deprecated, see Annex D. —end note ] The value computation of the ++ expression is sequenced before the modification of the operand object. With respect to an indeterminately-sequenced function call, the operation of postfix ++ is a single evaluation. [Note: Therefore, a function call shall not intervene between the lvalue-to-rvalue conversion and the side effect associated with any single postfix ++ operator. —end note ] The result is a prvalue

[ri_aje]

前加返回左值，后加返回右值。 decltype 对左值的结果就是引用，对右值就是非引用类型。

[fefe82]

续1楼： 4 For an expression e, the type denoted by decltype(e) is defined as follows: (4.1) — if e is an unparenthesized id-expression or an unparenthesized class member access (5.2.5), decltype(e) is the type of the entity named by e. If there is no such entity, or if e names a set of overloaded functions, the program is ill-formed; (4.2) — otherwise, if e is an xvalue, decltype(e) is T&&, where T is the type of e; (4.3) — otherwise, if e is an lvalue, decltype(e) is T&, where T is the type of e; (4.4) — otherwise, decltype(e) is the type of e. decltype(++i) var9 = i; //var9:int& ++i返回i的左值, (4.3) decltype(i++) var10; //var10:int i++返回右值, (4.4) , prvalue 不是 xvalue 。

[赵4老师]

关于C,C++表达式求值顺序 http://blog.csdn.net/fefe82/article/details/37833767，http://bbs.csdn.net/topics/370153775

[ztenv]

++i，返回的是自身加一的结果， i++，返回的是自身加一之前的临时值，

[paschen]

前者返回的是左值 后者返回的是右值

[paschen]

对于 decltype b) if the value category of expression is lvalue, then decltype yields T&; http://en.cppreference.com/w/cpp/language/decltype