C++ constexpr && consteval && constinit

前言

什么叫做函数运行于compile-time?非常简单,说的就是一个函数代码由编译器编译成机器代码,且直接在compile-time执行,我们知道函数有传参,如果参数在compile-time就知道了,那么函数在compile-time执行也就变得可能,假设参数只有少部分可以在compile-time所知,那么函数可能只有部分在compile-time执行

const vs constexpr

• const:

  • 首先const是runtime constant
  • const不能用于普通的函数,只能用于non-static member function

    static member function或者member data就是class里面定义的成员变量或者函数前面加上static关键字,不论class实例化多少次经此一份,且static member function/data没有this指针,可以在class外部直接访问和初始化,而non-static member function相反,只能由class对象初始化，如下

    #include 
    using namespace std;
    
    struct X {
    private:
    	int i;
    	static int si; //static member data
    public:
    	void set_i(int arg) { i = arg; }
    	static void set_si(int arg) { si = arg; }
    
    	void print_i() { //non-static member function
    		cout << "Value of i = " << i << endl;
    		cout << "Again, value of i = " << this->i << endl;
    	}
    
    	static void print_si() { //static member function
    		cout << "Value of si = " << si << endl;
    		cout << "Again, value of si = " << this->si << endl; // error
    	}
    
    };
    
    int X::si = 77;       // Initialize static data member
    
    int main() {
    	X xobj;
    	xobj.set_i(11);
    	xobj.print_i();
    
    // static data members and functions belong to the class and
    // can be accessed without using an object of class X
    	X::print_si();
    	X::set_si(22);
    	X::print_si();
    }
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• constexpr:

  • compile-time constan
  • 可以用于variable和function
  • 所有的constexpr变量是const的,constexpr member function并没有意味着const

constexpr vs consteval

• constexpr上面介绍了,constexpr 变量是compile-time，而constexpr 函数则是询问编译器,这个constexpr函数可否在compile-time执行,如果可以最好,不行就放到run-time执行,constexpr是C++ 11的特性
• consteval首先要说明的是consteval是c++ 20的标准,并且consteval不能声明变量,换句话说consteval只能声明函数,且consteval函数是告诉编译器,将这个函数放到compile-time执行,如果不行就直接报错

constexpr int foo (int x) { return x; }
consteval int bar (int x) { return x; } //一定会在compile-time中执行,否则就报错
consteval int v1 = 5; //error!!!

int main(){
	foo(42);
	int i = 5;
	foo(5);
	bar(42);
	bar(i);
}
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