C++核心準(zhǔn)則ES.28: 使用lambda表達(dá)式進(jìn)行變量的復(fù)雜初始化,特別...

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

ES.28: 使用lambda表達(dá)式進(jìn)行變量的復(fù)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(shí)變量，而不是生成一個(gè)不必要的非局部但卻不會(huì)重用的函數(shù)。它也可以用于應(yīng)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（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，將條件壓縮為一個(gè)由可選項(xiàng)（例如枚舉）構(gòu)成的簡(jiǎn)單集合并且不要將選擇和初始化混用。

Enforcement（實(shí)施建議）

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

很難。最好是啟發(fā)式的。尋找沒有初始化的變量的后面跟著為其賦值的循環(huán)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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