tips for constexpr and Constant Expressions

最新推荐文章于 2025-05-13 22:10:49 发布

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最新推荐文章于 2025-05-13 22:10:49 发布

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分类专栏： C++ 文章标签： constexpr constant expression

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本文深入探讨了C++中常量表达式的概念，包括其特性、为何不能在编译时常量表达式中使用普通变量，以及不同类型的常量表达式如‘低级常量’和‘高级常量’的区别。通过实例解析，帮助开发者掌握常量表达式的正确使用方法。

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A constant expression is an expression whose value cannot change and that can be evaluated at compile time.

A typical characteristic: the constant express will be evaluated at compile time, not at run time.

So when we use it and pay attention to it.. If we are implicitly to know what it is, don't use it.

for instant：

#include <iostream>

int main()
{
int m = 10;
constexpr int m2 = m;
std::cout << m2 << std::endl;
return 0;
}

when we compile it, it can't work it out.? Why?

It's easy to explain it. Because of the "m" variables are not usable when compiling. The compiler don't recognize it..

your compiler will tell you it's not usable in constant expression. Mine tell me that.. But if we told the compiler that "m" is a ”const int“

It will work out or to be compiled successfully..

so let we made some changes:

#include <iostream>
int main()
{
const m = 10;
constexpr int m2 = m;
std::cout << m2 << std::endl;
return 0;
}

I'm a cookie. There are a lot of concepts in the C++ world. It baffles me...

for example: const definition contain the "low-level const" and "top-level const". if they are both existed in the expression or statements, we can igonre the "top-level const".

How to distinguish it? C++ prime tells me that " Top-level const can appear in any object type, ...., Low-level const appears in the base type of compound types such as pointers or references. Note the pointer types, unlike most other types, can have both top-level and low-level const independently."

Another words: "... Like an object defined outside any function, these special local objects also have fixed addresses. Therefore, a constexpr reference may be bound to, and a constexpr pointer may address, such variables. "