ES.28: Use lambdas for complex initialization, especially of const variables
ES.28: 使用lambda表達式進行變量的複雜初始化，特別是常量變量
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.
這種方式漂亮地封裝了局部初始化，包括清理只在初始化過程中需要的臨時變量，而不是生成一個不必要的非局部但卻不會重用的函數。它也可以用於應該是常量但卻需要某些初始化處理的變量初始化.
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（範例）
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，將條件壓縮爲一個由可選項（例如枚舉）構成的簡單集合並且不要將選擇和初始化混用。
Enforcement（實施建議）
Hard. At best a heuristic. Look for an uninitialized variable followed by a loop assigning to it.
很難。最好是啓發式的。尋找沒有初始化的變量的後面跟着爲其賦值的循環的情況.
原文鏈接
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es28-use-lambdas-for-complex-initialization-especially-of-const-variables
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