在循环中重新声明for循环变量时出错 [英] Error redeclaring a for loop variable within the loop

问题描述

考虑以下C程序片段:

for(int i = 0; i < 5; i++)
{
    int i = 10;  // <- Note the local variable

    printf("%d", i); 
}    

它编译时没有任何错误，并且在执行时会给出以下输出:

It compiles without any error and, when executed, it gives the following output:

1010101010

但是，如果我在C ++中编写了类似的循环:

But if I write a similar loop in C++:

for(int i = 0; i < 5; i++)
{
     int i = 10;

     std::cout << i; 
}

编译失败，并显示以下错误:

The compilation fails with this error:

prog.cc:7:13: error: redeclaration of 'int i'  
     int i = 10;  
         ^  
prog.cc:5:13: note: 'int i' previously declared here  
     for(int i = 0; i < 5; i++)  
             ^   

为什么会这样?

推荐答案

这是因为C和C ++语言对于在嵌套在for循环中的作用域中重新声明变量具有不同的规则:

This is because C and C++ languages have different rules about re-declaring variables in a scope nested in a for loop:

• C++将i放在循环的主体范围内，因此第二个int i = 10是重新声明，这是禁止的
• C允许在for循环中的作用域中进行重新声明；最里面的变量"wins"

• C++ puts i in the scope of loop's body, so the second int i = 10 is a redeclaration, which is prohibited
• C allows redeclaration in a scope within a for loop; innermost variable "wins"

以下是正在运行的C程序和在主体内部打开嵌套作用域可修复编译错误(演示):

Opening a nested scope inside the body fixes the compile error (demo):

for (int i =0 ; i != 5 ; i++) {
    {
        int i = 10;
        cout << i << endl;
    }
}

现在for标头中的i和int i = 10处于不同的作用域，因此允许运行该程序.

Now i in the for header and int i = 10 are in different scopes, so the program is allowed to run.

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