Preface ： This article is mainly used for personal review , Pursue simplicity , Thank you for your reference 、 Communication and handling , It may be revised and improved in the future .

Because it's a personal review , There will be partial compression and omission .

One 、 Inline function

C Language in order to avoid small functions to establish stack frames , Macro functions are provided , It will be carried out in the pretreatment stage

since C Language has been solved , Why? C++ Also provide inline Function? ？（ Disadvantages of macro functions ）

1. Macro functions do not support debugging

2. Macro function syntax is complex , It's easy to make mistakes

3. Macro functions do not have type safety checks

C++ Small functions that are frequently called are recommended , Defined as inline, Will expand where called , No stack frame overhead

Can all functions be inlined ？

For example, here is a sort Function has 100 Line instruction , Yes 10 Place to call , How many instructions is the total ？

Yes 110 strip .

If it becomes an inline function , How many ？

100*10, Yes 1000 Orders .

What does it mean to have more instructions ？

More instructions means that the compiled executable program becomes larger , It will make the experience of the people who install the software worse , The memory consumption of the executing program becomes more

Conclusion ： Small functions that are called frequently , The suggestion is defined as inline

Characteristics of inline functions ：

1. inline It's a way of trading space for time , Save the cost of calling the function . So the code is long or has loops / Recursive functions are not suitable for inline functions .

2. inline Just a suggestion for the compiler , The compiler automatically optimizes , If defined as inline The function body of is too large or recursive , The compiler optimizes without inline .
3. inline Separation of declaration and definition is not recommended , Separation can lead to link errors . because inline Be unfolded , There is no function address , The link will not be found .

The root cause of link errors ： Inline function has no address ！！！ 

Two 、auto

Type declaration auto, According to a The type of automatic derivation b The type of

（auto*）malloc（） This is not allowed

#include <map>
#include <string>
#include <iostream>
using namespace std;

int main()
{

    int a = 10;
    //int b = a;
    auto b = a;// The type is declared as auto, According to a The type of automatic derivation b The type of 
    map<string, string> m;
    //map<string, string>::iterator it = m.begin();
    // Here we can base on m.begin() Automatic derivation it The type is map<string, string>::iterator
    auto it = m.begin();

    return 0;
}

 auto There are several places that cannot be used ：

1.auto Cannot be used as an argument to a function

auto Cannot be a parameter type , Because the compiler can't a The actual type of

void test(auto a)
{}

2.auto Cannot be used directly to declare arrays

void test()
{
    int a[] = {1,2,3};
    auto b[] = {4,5,6};
    // Can you do this ？ When creating an array ,auto b[0] = a[1];
        auto a = 4.1,b = 4;// You can't either 
}

3.auto In practice, the most common advantage usage is to talk about morality later C++11 New for loop , also lambda Expressions, etc

auto The real significance lies in supporting C++11 The new grammatical scope of for, The new syntax traverses , It's simpler , Array is ok , Containers can also

 e Your name can be changed

  Be careful , In this case, the scope cannot be used for

void test(int arr[])
{

    for(auto& e : arr)
    {
        cout << e << endl;
    }


}

Range for Conditions of use
1. for The scope of loop iteration must be determined
For arrays , Is the range of the first and last element in the array ; For a class , Should provide begin and end Methods ,begin and end Namely for Scope of loop iteration . Be careful ： There is a problem with the following code , because for The scope of is uncertain ,int array[] It is not an array , It's a pointer , So no

void TestFor(int array[]) 
{
for(auto& e : array) 
cout<< e <<endl;
}

2. iteration
The object of the ++ and == The operation of .( About iterators , I'll talk about it later )

3、 ... and 、nullptr and NULL

stay C++ We use nullptr, because C++ in NULL It's actually being define become 0 Of , Is such a macro , It's not a pointer

void f(int) 
{
    cout<<"f(int)"<<endl; 
}
void f(int*) 
{
    cout<<"f(int*)"<<endl; 
}
int main()
{
    f(0);
    f(NULL);
    f((int*)NULL); 
    return 0;
}

The purpose of the program is to pass f(NULL) Call the pointer version of f(int*) function , But because of NULL Defined as 0, Therefore, it is contrary to the original intention of the procedure . 
stay C++98 in , Literal constants 0 It can be an integer number , It can also be a typeless pointer (void*) Constant , But the compiler by default
Think of it as an integer constant , If you want to use it as a pointer , It must be forced (void *)0.