Lazily initialized data member: c++11, thread safe, const correct.

int_with_lazy_string_repr.cc

#include <string>

class Int {
 public:
  explicit Int(int i) : i_(i) {
    static_assert(sizeof(lazy_) == sizeof(void*),
                  "lazy members take up only one pointer's worth of space");
  }

  // naive string constructing accessor
  std::string Str() const { return std::to_string(i_); }

  // lazily accessor
  const std::string& LazyStr() const { return *lazy_; }

 private:
  int i_;

  struct L : lazy<std::string, L> {
    std::string get() const { return Member(&Int::lazy_).Str(); }
  } lazy_;
};

constexpr int kIntValue = 841123325;

static void Baseline(benchmark::State& state) {
  const Int i(kIntValue);
  for (auto _ : state) {
    auto created_string = i.Str();
    benchmark::DoNotOptimize(created_string);
  }
}
BENCHMARK(Baseline);

static void Lazy(benchmark::State& state) {
  const Int i(kIntValue);
  for (auto _ : state) {
    const auto& lazy_string_ref = i.LazyStr();
    benchmark::DoNotOptimize(lazy_string_ref);
  }
}
BENCHMARK(Lazy);

lazy.h

#include <atomic>
#include <cassert>
#include <cstdint>

template <typename Constructed, typename This>
class lazy {
 public:
  const Constructed& operator*() const { return *get_lazy(); }

  const Constructed* operator->() const { return get_lazy(); }

 protected:
  lazy() : ptr_(nullptr) {}

  template <typename Of>
  const Of& Member(This Of::*member) const {
    auto offset = reinterpret_cast<intptr_t>(&(reinterpret_cast<Of*>(0)->*member));
    return *reinterpret_cast<const Of*>(reinterpret_cast<intptr_t>(cast()) - offset);
  }

 private:
  const Constructed* get_lazy() const {
    auto ptr = ptr_.load();

    if (ptr != nullptr) return ptr;

    ptr = new Constructed(cast()->get());

    Constructed* expected = nullptr;
    if (ptr_.compare_exchange_strong(expected, ptr)) {
      return ptr;
    }

    delete ptr;
    assert(expected != nullptr);
    return expected;
  }

  const This* cast() const { return static_cast<const This*>(this); }

  mutable std::atomic<Constructed*> ptr_;
};

LICENSE.txt

Boost Software License - Version 1.0 - August 17th, 2003

Permission is hereby granted, free of charge, to any person or organization
obtaining a copy of the software and accompanying documentation covered by
this license (the "Software") to use, reproduce, display, distribute,
execute, and transmit the Software, and to prepare derivative works of the
Software, and to permit third-parties to whom the Software is furnished to
do so, all subject to the following:

The copyright notices in the Software and this entire statement, including
the above license grant, this restriction and the following disclaimer,
must be included in all copies of the Software, in whole or in part, and
all derivative works of the Software, unless such copies or derivative
works are solely in the form of machine-executable object code generated by
a source language processor.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
DEALINGS IN THE SOFTWARE.

Extracted from @pitrou's Fingerprintable class