What details does C + + improve on the basis of C

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C++ Is in C Based on the improvement of language ,C A lot of grammar of language is in C++ Is still widely used , for example ：
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C++ Still use char、short、int、long、float、double Basic data type ;
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C++ Still use if...else、while、for、switch、break As a branching or cyclic structure ;
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C++ Still use +、-、*、/、%、++、--、<<、>> Equal operator ;
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C++ Still use typedef、#define、enum、struct etc. ;
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C++ Still use C The classic pointer in language （Pointer）, And the scope of use has increased , Even indispensable .
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Now let's talk about C++ What details have been improved .
Variable definition position
ANSI C Regulations , All local variables must be defined at the beginning of the function , There must be no other execution statement until the variable is defined .C99 The standard removes this restriction , however VC/VS Yes C99 It's not very positive , It still requires variables to be defined at the beginning of the function . Look at the code below ：
· #include <stdio.h>
· · int main(){
· · int a;
· · scanf("%d", &a);
· · int b;
· · scanf("%d", &b);
· · int c = a + b;
· · printf("%d\n", c);
· ·
· · return 0;
· · }
· Save the code to the source file main.c, So it can be in GCC、Xcode Compiled through , But in VC/VS I'll make a mistake next time .GCC、Xcode Yes C99 Our support is very good , You can define variables anywhere in a function ; but VC/VS Yes C99 There is little support for , All variables must be defined at the beginning of the function .

Save the above code to the source file again main.cpp, So it's in GCC、Xcode、VC/VS Can be compiled under . This is because C++ The original restrictions have been lifted , Variables are defined before they are used , It is not mandatory that all variables must be defined at the beginning of a function .
Note the suffix of the source file ,.c yes C The language code ,.cpp yes C++ Code , They compile differently .
Another benefit of lifting restrictions is , Can be in for A variable is defined in the control statement of the loop , Please see the following example ：
· #include <iostream>
· · using namespace std;
· ·
· · int sum(int n){
· · int total = 0;
· · // stay for The conditional statement of the loop defines variables inside i
· · for(int i=1; i<=n ;i++){
· · total += i;
· · }
· · return total;
· · }
· ·
· · int main(){
· · cout<<"Input a interge: ";
· · int n;
· · cin>>n;
· · cout<<"Total: "<<sum(n)<<endl;
· · return 0;
· · }
· Running results ：
Input a interge: 10
Total: 55

stay for Internally defined loop control variables i, It makes the code look more compact , And make i The scope of is limited to the whole of for Inside the loop statement （ Including cycle conditions and loop bodies ）, It reduces the probability of naming conflicts . In the subsequent coding process , I recommend this way of writing .

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Boolean type （bool）
stay C In language , There are two results of relational operation and logical operation , True and false ：0 Said the false , Not 0 Said really . for example ：
· #include <stdio.h>
· · int main(){
· · int a, b, flag;
· · scanf("%d %d", &a, &b);
· · flag = a > b;  //flag Save the result of relational operations
· · printf("flag = %d\n", flag);
· ·
· · return 0;
· · }
· Running results ：
10 20
flag = 0

C Language doesn't support grammar completely “ really ” and “ false ”, Just use 1 and 0 To represent the . This is in C++ Has been improved in ,C++ Added bool type , It usually occupies 1 Byte length .bool Type has only two values ,true and false：true Express “ really ”,false Express “ false ”. Please see the following example ：
· #include <iostream>
· · using namespace std;
· ·
· · int main(){
· · int a, b;
· · bool flag;  // Boolean variables
· · cin>>a>>b;
· · flag = a > b;
· · cout<<"flag = "<<flag<<endl;
· ·
· · return 0;
· · }
· 10 20
flag = 0

Unfortunately , stay C++ Use in cout Output bool The value of a variable is still a number 1 and 0 Express , instead of true or false.Java、PHP、JavaScript They also support Boolean types , But the output is true or false, I arbitrarily think it's more scientific .

You can also be explicit about bool Variable assignment , for example ：
· #include <iostream>
· · using namespace std;
· ·
· · int main(){
· · bool flag = true;
· · if(flag){
· · cout<<"true"<<endl;
· · }else{
· · cout<<"false"<<endl;
· · }
· ·
· · flag = false;
· · if(flag){
· · cout<<"true"<<endl;
· · }else{
· · cout<<"false"<<endl;
· · }
· ·
· · return 0;
· · }
· Running results ：
true
false
Be careful ,true and false yes C++ Keywords in .
In later coding , I recommend using bool Variables are used to represent logical operations 、 Relational operations and the values of switching variables