'？：'的返回类型（三元条件运算符） [英] Return type of '?:' (ternary conditional operator)

问题描述

为什么第一次返回引用？

  int x = 1; 
 int y = 2; 
（x> y？x：y）= 100; 
  

第二个不是？

  int x = 1; 
 long y = 2; 
（x> y？x：y）= 100; 
  

实际上，第二个根本没有编译 - 赋值左侧没有左值。 >

解决方案

表达式没有返回类型，它们有一个类型和 - 在最新的C ++标准中已知的一个值类别。 / p>

条件表达式可以是左值或右值。这是它的值类别。 （这是一个简化，在 C ++ 11 我们有lvalues，xvalues和prvalues。）

在非常宽泛和简单的术语中，左值指的是存储器中的对象，并且右值只是可能不一定附加到存储器中的对象的值。

一个赋值表达式赋值给一个对象，所以被赋值的东西必须是一个值。

对于条件表达式（？：）为左值第二个和第三个操作数必须是相同类型的 lvalue 。这是因为条件表达式的类型和值类别在编译时确定，并且无论条件是否为真，都必须适当。如果其中一个操作数必须转换为不同的类型以匹配另一个，则条件表达式不能是一个值，因为该转换的结果不是一个值。。

ISO / IEC 14882：2011参考资料：

3.10 [basic.lval ] Lvalues和Rvalues（关于值类别）

5.15 [expr.cond]条件运算符（条件表达式的类型和值类别的规则）

5.17 [expr.ass]赋值和复合赋值运算符（要求赋值的lhs必须是可修改的左值）

Why does the first return a reference?

int x = 1;
int y = 2;
(x > y ? x : y) = 100;

While the second does not?

int x = 1;
long y = 2;
(x > y ? x : y) = 100;

Actually, the second did not compile at all - "not lvalue left of assignment".

解决方案

Expressions don't have return types, they have a type and - as it's known in the latest C++ standard - a value category.

A conditional expression can be an lvalue or an rvalue. This is its value category. (This is somewhat of a simplification, in C++11 we have lvalues, xvalues and prvalues.)

In very broad and simple terms, an lvalue refers to an object in memory and an rvalue is just a value that may not necessarily be attached to an object in memory.

An assignment expression assigns a value to an object so the thing being assigned to must be an lvalue.

For a conditional expression (?:) to be an lvalue (again, in broad and simple terms), the second and third operands must be lvalues of the same type. This is because the type and value category of a conditional expression is determined at compile time and must be appropriate whether or not the condition is true. If one of the operands must be converted to a different type to match the other then the conditional expression cannot be an lvalue as the result of this conversion would not be an lvalue.

ISO/IEC 14882:2011 references:

3.10 [basic.lval] Lvalues and rvalues (about value categories)

5.15 [expr.cond] Conditional operator (rules for what type and value category a conditional expression has)

5.17 [expr.ass] Assignment and compound assignment operators (requirement that the l.h.s. of an assignment must be a modifiable lvalue)

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