从构造函数初始化结构

Question

我正在重新编写一些 C++，但我真的很生疏。我觉得如果我知道如何正确表达我的问题，我会找到一个快速的答案，但我仍然很感激你的帮助。

sanitycheck.cpp：

#include <string>    
using namespace std;

typedef struct STR_1 {    
  int val_a, val_b;

  STR_1 (int a, int b)
  { val_a = a; val_b = b; }    
} STR_1;

typedef struct STR_2{    
  string name;
  STR_1 myStr1;

  STR_2 (string n, STR_1 s)
  { name=n; myStr1 = s; }
} STR_2;

int main(){

  return 0;
} // end main

当我尝试使用 g++ -o sanitycheck ./test/sanitycheck.cpp 编译时，我得到以下信息，

./test/sanitytest.cpp: In constructor ‘STR_2::STR_2(std::string, STR_1)’:
./test/sanitytest.cpp:25:3: error: no matching function for call to ‘STR_1::STR_1()’
   { name=name; myStr1 = &s; }
   ^
./test/sanitytest.cpp:25:3: note: candidates are:
./test/sanitytest.cpp:11:3: note: STR_1::STR_1(int*, int*)
   STR_1 (int *a, int *b)
   ^
./test/sanitytest.cpp:11:3: note:   candidate expects 2 arguments, 0 provided
./test/sanitytest.cpp:7:16: note: STR_1::STR_1(const STR_1&)
 typedef struct STR_1 {
                ^
./test/sanitytest.cpp:7:16: note:   candidate expects 1 argument, 0 provided
./test/sanitytest.cpp:25:23: error: no match for ‘operator=’ (operand types are ‘STR_1’ and ‘STR_1*’)
   { name=name; myStr1 = &s; }
                       ^
./test/sanitytest.cpp:25:23: note: candidate is:
./test/sanitytest.cpp:7:16: note: STR_1& STR_1::operator=(const STR_1&)
 typedef struct STR_1 {
                ^
./test/sanitytest.cpp:7:16: note:   no known conversion for argument 1 from ‘STR_1*’ to ‘const STR_1&’

我不清楚的一点是，为什么STR_2 的STR_1 myStr1; 首先需要调用STR_1 构造函数？我不能用，初始化这两种类型吗，

int main()
{
    STR_1 bob = STR_1(5,6);
    STR_2 tom = STR_2('Tom',bob);
    return 0;
}

谢谢！

Answer 1

除非还不清楚从 cmets 到 OP 的链接：这里，

typedef struct STR_2{    
  string name;
  STR_1 myStr1;

  STR_2 (string n, STR_1 s)  // here the myStr1 default constructor is called
  { name=name; myStr1 = s; }
} STR_2;

要求STR_1 是默认可构造的。为了解决这个问题，您必须在构造函数的初始化列表中构造成员 STR_1 myStr1;：

  STR_2 (string n, STR_1 s) : name(n), myStr1(s) {}

DEMO

这会调用编译器生成的 STR_1 的复制构造函数，而不是默认构造函数（通过提供自定义构造函数来抑制自动生成）。

---

另一种选择是使用指向STR_1的指针：

typedef struct STR_2{    
  string name;
  std::unique_ptr<STR_1> myStr1;

  STR_2 (string n, STR_1 s)
  { name=name; myStr1 = std::make_unique<STR_1>(s); }  //just for the sake of explanation
                                                       //again, this would be better
                                                       //done in the initializer list
} STR_2;

然而，我会出于充分的理由放弃第一种选择。