Distinguish between C++ constant expressions, const, and constexpr (with code)

A constant expression is an expression whose value will not change and the calculation result can be obtained during the compilation process. It can be evaluated at compilation time expression.

Example 1:

#include <iostream>
using namespace std;

int main()
{
	
	const int a1 = 10;           // a1是常量表达式。

	const int a2 = a1 + 20;      // a2是常量表达式

	int a3 = 5;                  // a3不是常量表达式

	const int a4 = a3;           // a4不是常量表达式，因为a3程序的执行到达其所在的声明处时才初始化，所以变量a4的值程序运行时才知道。但编译没问题！

	return 0;
}

The above code can be compiled normally.

It shows that what is declared by const is not necessarily a constant expression!

The new C 11 standard stipulates that variables are allowed to be declared as constexpr type so that the compiler can verify whether the value of the variable is a constant expression. constexpr Specifier declaration can obtain the value of a function or variable at compile time. A variable declared as constexpr must be a constant and must be initialized with a constant expression.

Example 2:

#include <iostream>
using namespace std;

int main()
{
	
	const int a1 = 10;           // a1是常量表达式。

	const int a2 = a1 + 20;      // a2是常量表达式

	int a3 = 5;                  // a3不是常量表达式

	constexpr int a4 = a3;           // a4不是常量表达式，因为a3程序的执行到达其所在的声明处时才初始化，所以变量a4的值程序运行时才知道。编译报错！

	return 0;
}

constexpr int a4 = a3; Compilation will report an error!

Example 3:

#include <iostream>
using namespace std;

int main()
{
	
	const int a1 = 10;           // a1是常量表达式。
	const int a2 = a1 + 20;      // a2是常量表达式
	int a3 = 5;                  // a3不是常量表达式
	const int a4 = a3;           //   a4不是常量表达式，因为a3程序的执行到达其所在的声明处时才初始化，所以变量a4的值程序运行时才知道。编译报错！

	char arr1[a2];   // 没问题
	char arr2[&#39;y&#39;];  // 没问题，&#39;y&#39;的ASCII码为121，相当于 char arr2[121];

	char arr3[a4];   // 编译报错，因为a4不是常量表达式


	return 0;
}

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