C++核心准则C.180:使用联合体节约内存-轻识

C.180: Use unions to save memory

C.180:使用联合体节约内存

Reason(原因)

A union allows a single piece of memory to be used for different types of objects at different times. Consequently, it can be used to save memory when we have several objects that are never used at the same time.

联合体使用同一块内存管理在存在于不同时刻的不同类型的对象。也就是说，当不同的对象永远不会同时使用的时候，使用联合体可以节约内存。

Example(示例)

union Value {
    int x;
    double d;
};

Value v = { 123 };  // now v holds an int
cout << v.x << '\n';    // write 123
v.d = 987.654;  // now v holds a double
cout << v.d << '\n';    // write 987.654

But heed the warning: Avoid "naked" unions。

但是要留意这条准则:C.181:避免原始的联合体。

Example(示例)

// Short-string optimization

constexpr size_t buffer_size = 16; // Slightly larger than the size of a pointer

class Immutable_string {
public:
    Immutable_string(const char* str) :
        size(strlen(str))
    {
        if (size < buffer_size)
            strcpy_s(string_buffer, buffer_size, str);
        else {
            string_ptr = new char[size + 1];
            strcpy_s(string_ptr, size + 1, str);
        }
    }

    ~Immutable_string()
    {
        if (size >= buffer_size)
            delete string_ptr;
    }

    const char* get_str() const
    {
        return (size < buffer_size) ? string_buffer : string_ptr;
    }

private:
    // If the string is short enough, we store the string itself
    // instead of a pointer to the string.
    union {
        char* string_ptr;
        char string_buffer[buffer_size];
    };

    const size_t size;
};

Enforcement(实施建议)

???

原文链接：

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c180-use-unions-to-save-memory

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