ES.12: Do not reuse names in nested scopes
ES.12: 不要在嵌套作用域中重复使用同样的名称
Reason（原因）
It is easy to get confused about which variable is used. Can cause maintenance problems.
这会导致更难弄清楚到底哪个变量在使用。可能引起维护问题。
Example, bad（反面示例）
代码语言：javascript复制int d = 0;
// ...
if (cond) {
// ...
d = 9;
// ...
}
else {
// ...
int d = 7;
// ...
d = value_to_be_returned;
// ...
}
return d;
If this is a large if-statement, it is easy to overlook that a new d has been introduced in the inner scope. This is a known source of bugs. Sometimes such reuse of a name in an inner scope is called "shadowing".
这一个很大的if语句，很容易漏掉内部作用域引入了一个新变量d这个事实。这是有名的错误源之一。这种在内部作用域中重用名称的做法被称为“遮盖”。
Note（注意）
Shadowing is primarily a problem when functions are too large and too complex.
“遮盖”主要是在函数太大或者过于复杂时发生问题。
Example（示例）
Shadowing of function arguments in the outermost block is disallowed by the language:
处于最外侧的函数参数的遮盖问题是被语言禁止的。
代码语言：javascript复制void f(int x)
{
int x = 4; // error: reuse of function argument name
if (x) {
int x = 7; // allowed, but bad
// ...
}
}Example, bad（反面示例）
Reuse of a member name as a local variable can also be a problem:
重用成员名称作为局部变量同样会引起问题：
代码语言：javascript复制struct S {
int m;
void f(int x);
};
void S::f(int x)
{
m = 7; // assign to member
if (x) {
int m = 9;
// ...
m = 99; // assign to local variable
// ...
}
}
Exception（例外）
We often reuse function names from a base class in a derived class:
我们经常在派生类中重用基类的函数名：
代码语言：javascript复制struct B {
void f(int);
};
struct D : B {
void f(double);
using B::f;
};
This is error-prone. For example, had we forgotten the using declaration, a call d.f(1) would not have found the int version of f.
这容易引发错误。例如，如果我们忘记using声明，调用d.f(1)时就无法发现f函数的整数版本。
??? Do we need a specific rule about shadowing/hiding in class hierarchies?
我们是否需要定义一个特别的适用于类继承情况下的遮盖/隐藏规则？
Enforcement（实施建议）
- Flag reuse of a name in nested local scopes
- 标记嵌套作用域中的名称重用。
- Flag reuse of a member name as a local variable in a member function
- 标记使用成员名称定义局部变量的情况。
- Flag reuse of a global name as a local variable or a member name
- 标记使用全局名称定义局部变量和成员名称的情况。
- Flag reuse of a base class member name in a derived class (except for function names)
- 标记派生类中重用基类名称的情况（函数名称除外）
原文链接
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es12-do-not-reuse-names-in-nested-scopes
