应该在嵌套块内定义变量.

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12345678910111213141516171819#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.：

12345678910111213141516#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;}

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