什么时候应该在C ++ 11中使用constexpr能力？

问题描述：

在我看来，有一个总是返回5的函数打破或稀释调用函数的含义。必须有一个原因，或者需要这个能力，否则它不会在C ++ 11中。为什么会有？

It seems to me that having a "function that always returns 5" is breaking or diluting the meaning of "calling a function". There must be a reason, or a need for this capability or it wouldn't be in C++11. Why is it there?

// preprocessor.
#define MEANING_OF_LIFE 42

// constants:
const int MeaningOfLife = 42;

// constexpr-function:
constexpr int MeaningOfLife () { return 42; }

在我看来，如果我写了一个函数返回一个字面值，代码审查，有人会告诉我，我应该声明一个常量值，而不是写返回5。

It seems to me that if I wrote a function that return a literal value, and I came up to a code-review, someone would tell me, I should then, declare a constant value instead of writing return 5.

答

假设它做一些更复杂的事情。

Suppose it does something a little more complicated.

constexpr int MeaningOfLife ( int a, int b ) { return a * b; }

const int meaningOfLife = MeaningOfLife( 6, 7 );

现在你有一些东西可以被评估为一个常量，同时保持良好的可读性，

Now you have something that can be evaluated down to a constant while maintaining good readability and allowing slightly more complex processing than just setting a constant to a number.

它基本上提供了一个很好的辅助可维护性，因为它变得更加明显你在做什么。取 max（a，b）例如：

It basically provides a good aid to maintainability as it becomes more obvious what you are doing. Take max( a, b ) for example:

template< typename Type > constexpr Type max( Type a, Type b ) { return a < b ? b : a; }

它是一个很简单的选择，但它的意思是，如果你调用

Its a pretty simple choice there but it does mean that if you call max with constant values it is explicitly calculated at compile time and not at runtime.

另一个很好的例子是 DegreesToRadians 函数。每个人都发现比弧度更容易阅读。虽然你可能知道180度是以弧度，它更清楚地写如下：

Another good example would be a DegreesToRadians function. Everyone finds degrees easier to read than radians. While you may know that 180 degrees is in radians it is much clearer written as follows:

const float oneeighty = DegreesToRadians( 180.0f );

这里有很多好处：

http://www.informit.com/guides/content.aspx?g=cplusplus& seqNum = 315

什么时候应该在C ++ 11中使用constexpr能力？

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