I have the following two files

foobar.h

#ifndef FOOBAR_H
#define FOOBAR_H

#include <cstring>

class Foo {
public:
  int x;
  Foo(int x);
};
    
class Bar {
public:
  char* name;
  Foo foo;
  Bar(Foo foo);
};

#endif // FOOBAR_H

and foobar.cpp

#include "foobar.h"

Foo::Foo(int x) {
    // Do something
}

Bar::Bar(Foo foo) {
    // Do something
}

Attempting to compile these with g -c foobar.cpp -o foobar.o results in the following error:

foobar.cpp: In constructor ‘Bar::Bar(Foo)’:
foobar.cpp:9:17: error: no matching function for call to ‘Foo::Foo()’
 Bar::Bar(Foo foo) {
                 ^
foobar.cpp:5:1: note: candidate: ‘Foo::Foo(int)’
 Foo::Foo(int x) {
 ^~~
foobar.cpp:5:1: note:   candidate expects 1 argument, 0 provided
In file included from foobar.cpp:1:
foobar.h:5:7: note: candidate: ‘constexpr Foo::Foo(const Foo&)’
 class Foo {
       ^~~
foobar.h:5:7: note:   candidate expects 1 argument, 0 provided
foobar.h:5:7: note: candidate: ‘constexpr Foo::Foo(Foo&&)’
foobar.h:5:7: note:   candidate expects 1 argument, 0 provided

As far as I understand the output of g is that it requires me to have a default constructor for foo. Why? I wish to pass a foo object to the constructor of bar, why would it need to invoke a default constructor anywhere? I do not want to have a no-args constructor anyways, since I NEED that x has a specific user defined value.

CodePudding user response：

You are trying to default construct a Foo here:

Bar::Bar(Foo foo) {
    // the member variable `foo` would have been default constructed here

    // Do something
}

But Foo doesn't have a default constructor. One possible solution would be to initialize it in the member initializer list:

Bar::Bar(Foo foo) : foo(std::move(foo)) { // now uses the move constructor instead
    // Do something
}