C++核心準(zhǔn)則ES.34:不要定義C風(fēng)格的可變參數(shù)函數(shù)

ES.34: Don't define a (C-style) variadic function

ES.34:不要定義C風(fēng)格的可變參數(shù)函數(shù)

Reason（原因）

Not type safe. Requires messy cast-and-macro-laden code to get working right.

這種方式不是類(lèi)型安全的。需要繁雜的類(lèi)型轉(zhuǎn)換和宏裝載代碼來(lái)保證正確動(dòng)作。

Example（示例）

#include 

// "severity" followed by a zero-terminated list of char*s; write the C-style strings to cerr
void error(int severity ...)
{
    va_list ap;             // a magic type for holding arguments
    va_start(ap, severity); // arg startup: "severity" is the first argument of error()

    for (;;) {
        // treat the next var as a char*; no checking: a cast in disguise
        char* p = va_arg(ap, char*);
        if (!p) break;
        cerr << p << ' ';
    }

    va_end(ap);             // arg cleanup (don't forget this)

    cerr << '\n';
    if (severity) exit(severity);
}

void use()
{
    error(7, "this", "is", "an", "error", nullptr);
    error(7); // crash
    error(7, "this", "is", "an", "error");  // crash
    const char* is = "is";
    string an = "an";
    error(7, "this", "is", an, "error"); // crash
}

Alternative: Overloading. Templates. Variadic templates.

可選項(xiàng)：重載，模板，可變參數(shù)模板。

#include 

void error(int severity)
{
    std::cerr << '\n';
    std::exit(severity);
}

template 
constexpr void error(int severity, T head, Ts... tail)
{
    std::cerr << head;
    error(severity, tail...);
}

void use()
{
    error(7); // No crash!
    error(5, "this", "is", "not", "an", "error"); // No crash!

    std::string an = "an";
    error(7, "this", "is", "not", an, "error"); // No crash!

    error(5, "oh", "no", nullptr); // Compile error! No need for nullptr.
}

Note（注意）

This is basically the way?printf?is implemented.

這是實(shí)現(xiàn)printf的基本方法。

Enforcement（實(shí)施建議）

• Flag definitions of C-style variadic functions.

• 標(biāo)記定義了C風(fēng)格可變參數(shù)函數(shù)的情況。
  

• Flag?#include ?and?#include

• 標(biāo)記代碼中包含#include 和?#include 的情況。

原文鏈接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#-es34-dont-define-a-c-style-variadic-function

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