C++核心准则​NR.1：不要坚持所有声明都应该放在函数顶部

NR.1: Don't insist that all declarations should be at the top of a function

NR.1：不要坚持所有声明都应该放在函数顶部

Reason（原因）

The "all declarations on top" rule is a legacy of old programming languages that didn't allow initialization of variables and constants after a statement. This leads to longer programs and more errors caused by uninitialized and wrongly initialized variables.

“将所有所有声明放在最上面”规则是旧编程语言的遗产，该编程语言（就是C语言，译者注）不允许在语句后初始化变量和常量。 这将导致更长的程序，更多由于变量未初始化或错误初始化引发的错误。

Example, bad（反面示例）

int use(int x)
{
    int i;
    char c;
    double d;

    // ... some stuff ...

    if (x < i) {
        // ...
        i = f(x, d);
    }
    if (i < x) {
        // ...
        i = g(x, c);
    }
    return i;
}

The larger the distance between the uninitialized variable and its use, the larger the chance of a bug. Fortunately, compilers catch many "used before set" errors. Unfortunately, compilers cannot catch all such errors and unfortunately, the bugs aren't always as simple to spot as in this small example.

未初始化变量与使用该变量的代码之间的距离越大，发生错误的机会越大。幸运的是，编译器可以捕获许多“设置前使用”错误。不幸的是，编译器无法捕获所有此类错误，这些错误并不总是像这个小例子中那样容易发现。

Alternative（替代方案）

• Always initialize an object
• 保证所有对象被初始化
• ES.21: Don't introduce a variable (or constant) before you need to use it
• ES.21: 不要在不需要时引入变量（或常量）

原文链接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#nr1-dont-insist-that-all-declarations-should-be-at-the-top-of-a-function