C++核心准则ES.28: 使用lambda表达式进行变量的复杂初始化，特别是常量变量

ES.28: Use lambdas for complex initialization, especially of const variables

ES.28: 使用lambda表达式进行变量的复杂初始化，特别是常量变量

Reason（原因）

It nicely encapsulates local initialization, including cleaning up scratch variables needed only for the initialization, without needing to create a needless non-local yet non-reusable function. It also works for variables that should be const but only after some initialization work.

这种方式漂亮地封装了局部初始化，包括清理只在初始化过程中需要的临时变量，而不是生成一个不必要的非局部但却不会重用的函数。它也可以用于应该是常量但却需要某些初始化处理的变量初始化.

Example, bad（反面示例）

widget x;   // should be const, but:
for (auto i = 2; i <= N; ++i) {          // this could be some
    x += some_obj.do_something_with(i);  // arbitrarily long code
}                                        // needed to initialize x
// from here, x should be const, but we can't say so in code in this style

Example, good（范例）

const widget x = [&]{
    widget val;                                // assume that widget has a default constructor
    for (auto i = 2; i <= N; ++i) {            // this could be some
        val += some_obj.do_something_with(i);  // arbitrarily long code
    }                                          // needed to initialize x
    return val;
}();

Example（示例）

string var = [&]{
    if (!in) return "";   // default
    string s;
    for (char c : in >> c)
        s += toupper(c);
    return s;
}(); // note ()

If at all possible, reduce the conditions to a simple set of alternatives (e.g., an enum) and don't mix up selection and initialization.

如果可能，将条件压缩为一个由可选项（例如枚举）构成的简单集合并且不要将选择和初始化混用。

Enforcement（实施建议）

Hard. At best a heuristic. Look for an uninitialized variable followed by a loop assigning to it.

很难。最好是启发式的。寻找没有初始化的变量的后面跟着为其赋值的循环的情况.

链接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es28-use-lambdas-for-complex-initialization-especially-of-const-variables