C++核心准则SL.con.4：不要对不能直接拷贝的参数使用memset或memcpy

SL.con.4: don't use memset or memcpy for arguments that are not trivially-copyable

SL.con.4：不要对不能直接拷贝的参数使用memset或memcpy

Reason（原因）

Doing so messes the semantics of the objects (e.g., by overwriting a vptr).

这样做会搞乱对象的语义（例如覆盖虚函数指针）。

Note（注意）

Similarly for (w)memset, (w)memcpy, (w)memmove, and (w)memcmp

(w)memset, (w)memcpy, (w)memmove, and (w)memcmp的情况也类似。

Example（示例）

struct base {
    virtual void update() = 0;
};

struct derived : public base {
    void update() override {}
};


void f(derived& a, derived& b) // goodbye v-tables
{
    memset(&a, 0, sizeof(derived));
    memcpy(&a, &b, sizeof(derived));
    memcmp(&a, &b, sizeof(derived));
}

Instead, define proper default initialization, copy, and comparison functions

正确的方法是定义适当的默认初始化，拷贝和比较函数

void g(derived& a, derived& b)
{
    a = {};    // default initialize
    b = a;     // copy
    if (a == b) do_something(a, b);
}

Enforcement（实施建议）

• Flag the use of those functions for types that are not trivially copyable 标记使用针对不可简单复制的类型使用上述函数的情况。

TODO Notes（待办记录）:

• Impact on the standard library will require close coordination with WG21, if only to ensure compatibility even if never standardized. 对标准库的冲击要求和WG21进行紧密合作，如果只确保兼容性，恐怕永远也无法标准化。
• We are considering specifying bounds-safe overloads for stdlib (especially C stdlib) functions like memcmp and shipping them in the GSL. 我们正在考虑为类似memcmp的stdlib（特别是C标准库）函数定义重载版本并发布到GSL中。
• For existing stdlib functions and types like vector that are not fully bounds-checked, the goal is for these features to be bounds-checked when called from code with the bounds profile on, and unchecked when called from legacy code, possibly using contracts (concurrently being proposed by several WG21 members). 对于存在的没有完全进行边界检查的标准库函数和类型，例如vector，目标是被边界准则群组有效的代码调用时可以进行边界检查，被历史代码调用时不检查。实现方式有可能是使用契约（同时被多位WG21成员建议）

原文链接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#slcon4-dont-use-memset-or-memcpy-for-arguments-that-are-not-trivially-copyable