Initializing a struct from a constructor - c++

Question

I'm getting back into writing some C++ and I'm honestly rusty. I get the feeling I'd find a quick answer to my question if I just knew how to phrase it properly, but still I'd appreciate your help.
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
When I try to compile with g++ -o sanitycheck ./test/sanitycheck.cpp I get the following,
./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&’
One thing I'm not clear on is why would STR_1 myStr1; of STR_2 need to call the STR_1 constructor in the first place? Couldn't I initialize both types with,
int main()
{
STR_1 bob = STR_1(5,6);
STR_2 tom = STR_2('Tom',bob);
return 0;
}
Thanks!

Answer 1

Unless it is not already clear from the link in the comments to the OP: this here,
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;
requires STR_1 to be default constructible. In order to work around, you have to construct the member STR_1 myStr1; in the constructor's initializer list:
STR_2 (string n, STR_1 s) : name(n), myStr1(s) {}
DEMO
This calls the compiler generated copy-constructor of STR_1 instead of the default constructor (the automatic generation of which is suppressed by providing a custom constructor).
Another option would be to use a pointer to STR_1:
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;
Yet, I would depart from the first alternative only for a good reason.