How to copy "partially" an object using a std::function? - c++

Question

Here's the code I have:
#include <iostream>
#include <functional>
struct Test {
struct Envelope {
const int x = 1;
int y = 2;
int z = 3;
};
Envelope mEnvelope;
struct Buffer {
Envelope mEnvelope;
} mBuffer;
std::function<Buffer()> func{[this] {
mBuffer.mEnvelope = mEnvelope;
return mBuffer;
}};
};
int main() {
Test test;
}
it says:
g++ -std=c++17 -Wall -pedantic -pthread main.cpp && ./a.out
main.cpp: In lambda function:
main.cpp:17:29: error: use of deleted function 'Test::Envelope& Test::Envelope::operator=(const Test::Envelope&)'
17 | mBuffer.mEnvelope = mEnvelope;
| ^~~~~~~~~
main.cpp:5:12: note: 'Test::Envelope& Test::Envelope::operator=(const Test::Envelope&)' is implicitly deleted because the default definition would be ill-formed:
5 | struct Envelope {
| ^~~~~~~~
main.cpp:5:12: error: non-static const member 'const int Test::Envelope::x', can't use default assignment operator
I've tried using a Copy Constructor:
Envelope(const Envelope &other) {
y = other.y;
}
or override the operator =
Envelope &operator=(Envelope &other) {
// self-assignment guard
if (this == &other) {
return *this;
}
y = other.y;
}
But the errors grown even more.
I need to copy only some "part" of the object.
This is just a test, of course the real object have lots of members fields, and some need to be ignored.
How to do it within a std::function<Buffer()>?

Answer 1

OK, since it might not be obvious what the problem is:
A copy assignment operator should usually have a const& parameter, not a & parameter.
You should then also provide the copy constructor of Envelope as you showed. First of all it has different behavior than the implicitly-generated one (which would copy over all the elements, not only y) and the generation of the implicit copy constructor when there is a user-defined copy assignment has been deprecated since C++11 and will probably be removed in a future standard iteration.
Then you will need to default the default constructor as well, since you have a user-defined constructor now:
Envelope() = default;
Furthermore, your copy assignment operator is declared to return a Envelope& (as it should), but you forgot to actually put a return statement in it at its end, so executing it will cause undefined behavior as is:
return *this;

Answer 2

You can create your own copy ctor/operator in order to only copy the info you want:
#include <iostream>
#include <functional>
struct Test {
typedef struct {
const int x;
int y;
int z;
} Envelope_t;
public:
Test():env({1,2,3}){}
Test(const Test & copy):env({copy.env.x,5,copy.env.z}) {}
Test& operator=(const Test& copy){
env.y=copy.env.y+7;
env.z=copy.env.z;
return *this;
}
void printValues() {
std::cout << "x:" << env.x << "\ny:" <<
env.y << "\nz:" << env.z << "\n\n";
}
private:
Envelope_t env;
};
int main() {
Test original;
original.printValues();
Test copyCtor(original);
copyCtor.printValues();
Test copyOp;
copyOp = copyCtor;
copyOp.printValues();
}