C++核心准则ES.34:不要定义C风格的可变参数函数

翻译

大连木匠 2022-07-30 00:10:25 博主文章分类：C++

文章标签 C++ 核心准则 C风格可变参数函数 #include 文章分类 后端开发

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

ES.34:不要定义C风格的可变参数函数

Reason（原因）

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

这种方式不是类型安全的。需要繁杂的类型转换和宏装载代码来保证正确动作。

Example（示例）

#include <cstdarg>

// "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.

可选项：重载，模板，可变参数模板。

#include <iostream>

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

template <typename T, typename... Ts>
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.
}