C++核心准则DS.22:没有合适的初始值就不要定义变量-轻识

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

DS.22:没有合适的初始值就不要定义变量

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.

可读性。限制变量可用的范围。不要冒设定前使用的风险。初始化通常比赋值更高效。

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存在一个代价不高的默认初始化，这段代码问题不大。否则，程序员可能特别想知道是否通过条件迷宫的所有路径都被覆盖了。如果不是，我们就遇到了一个设定前使用的错误。这是一个维护陷阱。

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

对于中等复杂度初始化器，包括常量，考虑使用lambda表达式实现。参见ES.28

Enforcement(实施建议)

Flag declarations with default initialization that are assigned to before they are first read.
标记包含默认初始化操作却在第一次使用之前赋值的情况。
Flag any complicated computation after an uninitialized variable and before its use.
标记任何定义了未初始化变量又在它被使用之前进行了复杂处理的qi

原文链接

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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