导致 sizeof() 为 32 的字符串结构中的开销是多少？
Most modern std::string implementations1 save very small strings directly on the stack in a statically sized char array instead of using dynamic heap storage. This is known as Small (or Short) String Optimisation (SSO). It allows implementations to avoid heap allocations for small string objects and improves locality of reference.
此外，还将有一个std::size_t成员保存字符串大小和指向实际的指针char贮存。
具体实现方式有所不同，但大致如下：
template <typename T>
struct basic_string {
char* begin_;
size_t size_;
union {
size_t capacity_;
char sso_buffer[16];
};
};
在典型的架构中sizeof (void*)= 8，这给了我们 32 字节的总大小。
1 The “big three” (GCC’s libstdc++ since version 5, Clang’s libc++ and MSVC’s implementation) all do it. Others may too.
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