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1. Enhanced detection of global variables
C++ has a stricter distinction between the detection and definition of global variables, which can be defined in C language global variables:
int a; //定义
int a; //声明
int a; //声明
In the C language, the first one is regarded as a definition, and all subsequent variables with the same name are regarded as declarations.
In C++, this expression is wrong. Must use extern to modify:
int a;
extern int a;
extern int a;
2. New Bool type
That is, the Boolean type, which has only two values: true and false
. The use of the Boolean type in C language requires the #include <stdbool.h>introduction of a header file before the bool flag= true;bool type can be used.
In C++, there is no need to include a header file, and the bool type can be defined directly.
3. Enhancement of trinocular calculation
There are trinocular operations in C language, such as finding the maximum value:
int a=1, b=9;
big= a>b?a:b;
This means that if a>b, take a and assign it to big, otherwise, assign b to big. That is to say, in the C language, the ternary operation returns the value of the expression, which is a constant.
In C++, the ternary operation returns the result variable of the expression, which is a variable that can be re-assigned and operated.
#include <iostream> //标准输入输出流
using namespace std; //声明一段命名空间 ，且必须定义在全局范围内
int main()
{
int a=1, b=9;
(a>b?a:b) = 999;
cout << "a为：" << a << endl;
cout << "b为：" << b << endl;
}
Here, the ternary operator returns the variable with the maximum value, that is, b, and then assigns 999 to b, so the final output b is 999.
4. References to pointers and constants (alias)
(Pointer reference)
Formula type &p= q;for pointer reference: type is the pointer type.
#include <iostream> //标准输入输出流
using namespace std; //声明一段命名空间 ，且必须定义在全局范围内
void fun(int* &p)
{
}
int main()
{
int *p= NULL;
fun(p);
}
(2) Constant reference The
alias of the constant is the reference to the constant. Simply put, adding a const before the reference is the reference const int &b= a;to the constant a, that is, the alias b.
5. Limitations of inline functions
In the following situations, the compiler may not compile Bingshu inline:
- There can be any form of circulation
- There cannot be too many conditional judgments
- Function body cannot be too large
- Cannot perform addressing operations on functions
6, the placeholder parameter of the function
To put it bluntly, it is a parameter that is used to occupy a position, and it may not be used in a function.
- The placeholder parameter needs to specify the type, which means the placeholder;
- When passing parameters, the value of the parameter must be passed in and cannot be omitted.
- Placeholder parameters can specify default values (default values)
#include <iostream> //标准输入输出流
using namespace std; //声明一段命名空间 ，且必须定义在全局范围内
void fun(int a, int)
{
cout << a << endl;
}
void fun2(int b, int=2)
{
cout << b << endl;
}
int main()
{
fun(99, 1);
fun2(88);
}
7. The usage of extern "C"
This usage is used to declare the function in the .h external file, which means that it is a declaration of a C file, not a C++ file.
For example, the .c file is like this:
declare in the .h file that extern "C" should be included
so that we can call the mul function in the external file normally in the C++ file:
#include <iostream> //标准输入输出流
using namespace std; //声明一段命名空间 ，且必须定义在全局范围内
//要调用外部文件需要先包含 头文件，也就是对ex.c文件进行声明的文件
#include "ex.h"
int main()
{
cout << mul(3, 4) << endl;
}