alf-p-steinbach

#include "Visitable.hpp"

struct Cat: Visitable
{
    void hunt_birds() { throw "Hunting birds..."; }
    void accept( Doer& doer ) override { doer.deal_with( *this ); }
};

alf-p-steinbach

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<assembly manifestVersion="1.0" xmlns="urn:schemas-microsoft-com:asm.v1">
  <assemblyIdentity type="win32" name="Demo of Windows API level coding" version="1.0.0.0"/>
  <application>
    <windowsSettings>
      <activeCodePage xmlns="http://schemas.microsoft.com/SMI/2019/WindowsSettings"
                >UTF-8</activeCodePage>
    </windowsSettings>
  </application>
  <dependency>

alf-p-steinbach

#include "draw_c_curve_on.hpp"      // draw_c_curve_on
namespace g = graphics;

#include "util-string_from.hpp"
using   util::string_from;

#include <iostream>
#include <string>
using   std::cout, std::endl, std::ostream,
        std::string;

alf-p-steinbach

#pragma once
// Based on a header generated with GIMP, using Courier New font size 11.
// 8x14 cells, ASCII range 32 through 127.

namespace char_bitmap {
    using Byte = unsigned char;

    const int   cell_width      = 8;
    const int   cell_height     = 14;
    const int   char_height     = 9;

alf-p-steinbach

#pragma once
#include <string_view>      // std::string_view

//---------------------------------------- Lenna image scaled down to 51x51:
namespace lenna {
    using std::string_view;

        const auto& data_literal = R"(
iVBORw0KGgoAAAANSUhEUgAAADMAAAAzCAIAAAC1w6d9AAAABGdBTUEAALGPC/xh
BQAAAAlwSFlzAAAOwwAADsMBx2+oZAAAABh0RVh0U29mdHdhcmUAcGFpbnQubmV0

alf-p-steinbach

alf-p-steinbach

Why COW was deemed ungood for std::string.

COW, short for copy on write, is a way to implement mutable strings so that creating strings and logically copying strings, is reduced to almost nothing; conceptually they become free operations like no-ops.

Basic idea: to share a data buffer among string instances, and only make a copy for a specific instance (the copy on write) when that instance's data is modified. The general cost of this is only an extra indirection for accessing the value of a string, so a COW implementation is highly desirable. And so the original C++ standard, C++98, and its correction C++03, had special support for COW implementations, and e.g. the g++ compiler's std::string implementations used COW.

So why was that support dropped in C++11?

In particular, would the same reason or reasons apply to a reference counted immutable string value class?