应该在嵌套块内定义变量.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
#include <iostream>
 
int main()
{ // outer block
    int apples(5); // here's the outer block apples
 
    if (apples >= 5) // refers to outer block apples
    { // nested block
        // no inner block apples defined
 
        apples = 10; // this now applies to outer block apples, even though we're in an inner block
 
        std::cout << apples; // print value of outer block apples
    } // outer block apples retains its value even after we leave the nested block
 
    std::cout << apples; // prints value of outer block apples
 
    return 0;
} // outer block apples destroyed

在这两个例子中，外嵌苹果不会受到嵌套嵌段苹果的影响。这两个程序之间的唯一区别是苹果表达式苹果= 10适用于。
阴影是一般应该避免的东西，因为它是相当混乱！
规则：避免在外部块中使用与变量相同的嵌套变量。
变量应该在最有限的范围内定义

例如，如果变量只在嵌套块中使用，则应该在嵌套块内定义变量.：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
#include <iostream>
 
int main()
{
    // do not define y here
 
    {
        // y is only used inside this block, so define it here
        int y(5);
        cout << y;
    }
 
    // otherwise y could still be used here
 
    return 0;
}