蓝桥ROS机器人之现代C++学习笔记2.1常量

Posted 2022-04-02 zhangrelay

 

nullptr

C++11 引入了 nullptr 关键字，专门用来区分空指针、0。

#include <iostream>
#include <type_traits>

void foo(char *);
void foo(int);

int main() 
    if (std::is_same<decltype(NULL), decltype(0)>::value)
        std::cout << "NULL == 0" << std::endl;
    if (std::is_same<decltype(NULL), decltype((void*)0)>::value)
        std::cout << "NULL == (void *)0" << std::endl;
    if (std::is_same<decltype(NULL), std::nullptr_t>::value)
        std::cout << "NULL == nullptr" << std::endl;

    foo(0);          // will call foo(int)
    // foo(NULL);    // doesn't compile
    foo(nullptr);    // will call foo(char*)
    return 0;


void foo(char *) 
    std::cout << "foo(char*) is called" << std::endl;

void foo(int i) 
    std::cout << "foo(int) is called" << std::endl;

 g++ -std=c++17 2.01.nullptr.cpp -o nullptr.out

---

constexpr

#include <iostream>
#define LEN 10

int len_foo() 
    int i = 2;
    return i;

constexpr int len_foo_constexpr() 
    return 5;


// error in c++11
// constexpr int fibonacci(const int n) 
//     if(n == 1) return 1;
//     if(n == 2) return 1;
//     return fibonacci(n-1) + fibonacci(n-2);
// 

// ok
constexpr int fibonacci(const int n) 
    return n == 1 || n == 2 ? 1 : fibonacci(n-1) + fibonacci(n-2);



int main() 
    char arr_1[10];                      // legal
    char arr_2[LEN];                     // legal

    int len = 10;
    // char arr_3[len];                  // illegal

    const int len_2 = len + 1;
    constexpr int len_2_constexpr = 1 + 2 + 3;
    // char arr_4[len_2];                // illegal, but ok for most of the compilers
    char arr_4[len_2_constexpr];         // legal

    // char arr_5[len_foo()+5];          // illegal
    char arr_6[len_foo_constexpr() + 1]; // legal
    
    // 1, 1, 2, 3, 5, 8, 13, 21, 34, 55
    std::cout << fibonacci(10) << std::endl;

    return 0;

 g++ -std=c++17 2.02.constexpr.cpp -o constexpr.out

如果需要都输出加个循环即可；

 

 

    for(int i=1;i<11;i++)
    
	std::cout << fibonacci(i) << std::endl;
    

---