1. std::string 的预留空间

#include <iostream>
#include <string>
#include <string.h>

using std::cout;
using std::endl;

/**
 * Note: 增加了奇怪的知识点:
 * 1. std::string 创建的每个对象，都会预留15个字节的空间；
 * 2. std::string(""), 预留了15个字节的空间;
 * (就像创建了一个空的仓库，仓库里面没放东西。但是, 如果其他人有仓库钥匙的话，依然可以使用这个仓库)
 * 
 * str.capacity(): 返回当前为字符串分配的存储空间大小，以字节表示。
 *
 * data(): const char* data() const noexcept;
 * c_str(): const char* c_str() const noexcept;
 * Both string::data and string::c_str are synonyms and return the same value.
 * 官方解释：data() 和 c_str() 是一样的
 * 
 *
*/

int main() {

    // init empty string
    std::string str("");
    std::cout << "size: "       << str.size()       << endl;
    std::cout << "length: "     << str.length()     << endl;
    std::cout << "capacity: "   << str.capacity()   << endl;
    std::cout << "max_size: "   << str.max_size()   << endl;
    std::cout << "data: "       << str.data()       << endl;
    std::cout << "Address of str: " << &str << endl;

    char *mch = (char*)str.data();
    std::cout << "mch: " << mch << endl;
    std::cout << "Address of mch: " << static_cast<void *>(mch) << endl;

    char tem[5] = "q3we"; // '\0'

    memcpy((void*)(str.c_str()), tem, 5);
    std::cout << "size: "       << str.size()       << endl;
    std::cout << "length: "     << str.length()     << endl;
    std::cout << "capacity: "   << str.capacity()   << endl;
    std::cout << "max_size: "   << str.max_size()   << endl;
    std::cout << "data: "       << str.data()       << endl;
    std::cout << "Address of str: " << &str << endl;

    return 0;
}

结果：
size: 0
length: 0
capacity: 15
max_size: 9223372036854775807
data: 
Address of str: 0x7ffc76fa61f0
mch: 
Address of mch: 0x7ffc76fa6200
size: 0
length: 0
capacity: 15
max_size: 9223372036854775807
data: q3we
Address of str: 0x7ffc76fa61f0