問題描述

考慮這個 C 程序片段:

for(int i = 0; i <5; i++){國際我= 10;//<- 注意局部變量printf("%d", i);}

它編譯沒有任何錯誤，并且在執(zhí)行時給出以下輸出:

1010101010

但是如果我用 C++ 寫一個類似的循環(huán):

for(int i = 0; i <5; i++){國際我= 10;std::cout <<一世;}

編譯失敗并出現(xiàn)此錯誤:

prog.cc:7:13: 錯誤:'int i' 的重新聲明國際我= 10;^prog.cc:5:13: 注意:'int i' 之前在這里聲明過for(int i = 0; i <5; i++)^

為什么會這樣?

這是因為 C 和 C++ 語言對于在嵌套在 for 循環(huán)中的范圍內重新聲明變量有不同的規(guī)則:

• C++ 把i 放在循環(huán)體的作用域內，所以第二個int i = 10 是重聲明，禁止莉>
• C 允許在 for 循環(huán)內的范圍內重新聲明；最里面的變量獲勝"

這是一個運行C程序的演示，以及一個C++ 程序無法編譯.

在正文中打開嵌套范圍修復了編譯錯誤(demo):

for (int i =0 ; i != 5 ; i++) {{國際我= 10;cout<<我＜＜結束;}}

現(xiàn)在for頭中的i和int i = 10在不同的范圍內，所以程序可以運行了.>

Consider this snippet of a C program:

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

    printf("%d", i); 
}    

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

1010101010

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

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

     std::cout << i; 
}

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++)  
             ^   

Why is this happening?

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

• 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"

Here is a demo of a running C program, and a C++ program failing to compile.

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;
    }
}

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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