C++核心準(zhǔn)則DS.22:沒有合適的初始值就不要定義變量

ES.22: Don't declare a variable until you have a value to initialize it with

DS.22:沒有合適的初始值就不要定義變量

Reason（原因）

Readability. Limit the scope in which a variable can be used. Don't risk used-before-set. Initialization is often more efficient than assignment.

可讀性。限制變量可用的范圍。不要冒設(shè)定前使用的風(fēng)險。初始化通常比賦值更高效。

Example, bad（反面示例）

string s;
// ... no use of s here ...
s = "what a waste";

Example, bad（反面示例）

SomeLargeType var;   // ugly CaMeLcAsEvArIaBlE

if (cond)   // some non-trivial condition
    Set(&var);
else if (cond2 || !cond3) {
    var = Set2(3.14);
}
else {
    var = 0;
    for (auto& e : something)
        var += e;
}

// use var; that this isn't done too early can be enforced statically with only control flow

This would be fine if there was a default initialization for?SomeLargeType?that wasn't too expensive. Otherwise, a programmer might very well wonder if every possible path through the maze of conditions has been covered. If not, we have a "use before set" bug. This is a maintenance trap.

如果SomeLargeType存在一個代價不高的默認(rèn)初始化，這段代碼問題不大。否則，程序員可能特別想知道是否通過條件迷宮的所有路徑都被覆蓋了。如果不是，我們就遇到了一個設(shè)定前使用的錯誤。這是一個維護(hù)陷阱。

For initializers of moderate complexity, including for?const?variables, consider using a lambda to express the initializer; see?ES.28.

對于中等復(fù)雜度初始化器，包括常量，考慮使用lambda表達(dá)式實現(xiàn)。參見ES.28

Enforcement(實施建議)

• Flag declarations with default initialization that are assigned to before they are first read.

• 標(biāo)記包含默認(rèn)初始化操作卻在第一次使用之前賦值的情況。
  

• Flag any complicated computation after an uninitialized variable and before its use.

• 標(biāo)記任何定義了未初始化變量又在它被使用之前進(jìn)行了復(fù)雜處理的qi
  

原文鏈接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es22-dont-declare-a-variable-until-you-have-a-value-to-initialize-it-with

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