Instantiating struct with string that has string.reserve()

Question

I want to pass a string with string.reserve(1000) and ensure that memory is allocated when I pass it into different structs. Is there a way to do this without explicitly doing it after every instantiation?

I want to avoid having to do this:

struct Prefix{
  std::string path;
};

int main() {
  std::string path = "hel";
  path.reserve(1000);
  std::cout << path.capacity() << "\n"; // 1000

  Prefix x = {path};
  std::cout << x.path.capacity() << "\n"; // 15
  x.path.reserve(1000);
  x.path += somestring;
  Prefix y = {x.path};
  std::cout << y.path.capacity() << "\n"; // 15
  y.reserve(1000);
}

songyuanyao · Accepted Answer · 2018-06-22 03:43:33Z

3

You have to do it explicitly, compiler is not required to copy the capacity too when copying std::string.

You can add a constructor to do it for you.

struct Prefix{
  std::string path;
  Prefix(const string& p) : path(p) { path.reserve(p.capacity()); }
};

edited Jun 22, 2018 at 3:43

answered Jun 22, 2018 at 3:37

songyuanyao

174k16 gold badges327 silver badges432 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

user18764 Over a year ago

What if the capacity is a variable? Would I have to use a class instead?

songyuanyao Over a year ago

@user18764 What do you mean about capacity is a variable ?

o11c · Accepted Answer · 2018-07-03 02:04:15Z

0

If you don't need the origin string again, you should move rather than copy.

Prefix x = {std::move(path)};

This isn't technically guaranteed to preserve the capacity, but it's hard to imagine a sane implementation where it wouldn't work. (And no, SSO doesn't affect this, it just enforces a minimum capacity at all times).

answered Jul 3, 2018 at 2:04

o11c

16.5k5 gold badges57 silver badges80 bronze badges

Collectives™ on Stack Overflow

Instantiating struct with string that has string.reserve()

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related