How to avoid repeating yourself for structs with default values

Question

#include <iostream>
#include <string>
#include <cassert>

struct Foo
{
  std::string bar = "standard";
  int x = 1;
  int y = 1;

  bool operator==(const Foo&) const = default;
  bool operator!=(const Foo&) const = default;

  void ResetToStandard()
  {
    bar = "standard";
    x = 1;
    y = 1;
  }  
};

int main(int argc, char* argv[])
{
  Foo f, g;
  g.bar = "Hello world!";
  g.x = 123;
  g.y = 234;
  g.ResetToStandard();
  assert(f == g);
}

In this sample code (very simplified real world code) I have a struct Foo whose fields have default values. There is also a ResetToStandard function that sets all field to their standard value, but all the values are repeated in this function.

Then I came up with this, which works fine.

void ResetToStandard()
{
  *this = Foo();
}

Is this pattern common? Is this the idiomatic way?

Not sure what is the best/idiomatic way, but you can remove the initializers from the members declarations, and call ResetToStandard() from a default ctor. — wohlstad
– wohlstad, Commented Jun 6 at 11:28
What do you expect about capacity of members (as std::string/std::vector)? manual reinitialization might kept the capacity, whereas assigning from new object will re-init capacity too. — Jarod42
– Jarod42, Commented Jun 6 at 11:29
Is this pattern common? It is the pattern I'd use. I do not use it commonly, because resetting an object to an initial state is not something I commonly need to do. It is idiomatic enough that my only point of concern is whether it is better to have a ResetToStandard() method or to have the callsite itself just do g = Foo{};. The method makes sense for a polymorphic class hierarchy. — Eljay
– Eljay, Commented Jun 6 at 11:58
@wohlstad Note that this can result in less efficient machine code for the default constructor (bar will be first default-initialized and then assigned-into): godbolt.org/z/xvrq4c68q. — Daniel Langr
– Daniel Langr, Commented Jun 6 at 13:40

Toby Speight · Accepted Answer · 2025-06-15 07:27:58Z

5

I would use *this = {} like this (to reassign this from default constructed object).
Which should use the move constructor.

#include <iostream>
#include <string>
#include <cassert>

struct Foo
{
    std::string bar = "standard";
    int x = 1;
    int y = 1;

    bool operator==(const Foo&) const = default;
    bool operator!=(const Foo&) const = default;

    void ResetToStandard()
    {
        *this = {};
    }
};

int main(int argc, char* argv[])
{
    Foo f, g;
    g.bar = "Hello world!";
    g.x = 123;
    g.y = 234;
    g.ResetToStandard();
    assert(f == g);
}

edited Jun 15 at 7:27

Toby Speight

32.4k58 gold badges83 silver badges118 bronze badges

answered Jun 6 at 14:34

Pepijn Kramer

14k4 gold badges12 silver badges27 bronze badges

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