C++ DLLs: how to create the corresponding include headers? - c++

Question

Problem
I have a C++ project and create a library A from it. If I now link another project B with this library A, I of course also have to provide an include path for A's headers, so I just use A's source folder. But A's headers contain symbols that aren't exported. I feel like this is not the correct way to do it, but don't know better. A specific thing that makes me feel like this is incorrect is that my IDE suggests the symbols that aren't exported.
I'd guess the solution would be to create an include folder besides the source folder where the same headers are in but only with the exported symbols. So at build-time, every symbol with PROJECTAPI should be automatically copied over to the corresponding headers in the include folder. But if I google, I don't find such a function for e.g. cmake.
So what would be the recommended way here? Is there a functionality to create such an include folder?
Example
example.cpp of project B
#include <A/main.hpp>
int main() {
ex::World w("Earth");
w.say_hello();
//IDE wouldn't see this as error: w.private();
}
main.cpp of project A
#include <iostream>
#include "main.hpp"
namespace ex {
void World::say_hello() {
std::cout << "Hello, World from " << m_name << std::endl;
}
World::World(std::string name)
: m_name(name)
{}
void World::hidden() {
std::cout << "Not exported" << std::endl;
}
}
main.hpp of project A
#include <string>
#ifndef PROJECTAPI
# ifdef example_EXPORTS
# define PROJECTAPI __declspec(dllexport)
# else
# define PROJECTAPI __declspec(dllimport)
# endif
#endif
namespace ex {
class World {
private:
std::string m_name;
public:
void PROJECTAPI say_hello();
PROJECTAPI World(std::string name);
void hidden();
};
}
Edit: private isn't a good method name

Answer 1

You are looking for the PIMPL idiom. "PIMPL" is short for "Pointer to IMPLementation". The idea is that, at the cost of a pointer indirection, you hide the implementation data and private methods in an inner class whose definition is opaque to the API consumer.
This approach is especially effective if you need to provide ABI stability.
Herb Sutter has a great GOTW on this here: https://herbsutter.com/gotw/_100/
Here is a full example that's close-ish to your code:
$ tree
.
├── CMakeLists.txt
├── include
│   └── world.h
├── main.cpp
└── src
├── world.cpp
└── world_priv.h
In ./include/world.h (the public header)
#ifndef WORLD_H
#define WORLD_H
#include <memory>
#include <string>
#include "world_export.h"
namespace ex {
class World {
public:
WORLD_EXPORT World(std::string name);
WORLD_EXPORT ~World() /* = default */;
void WORLD_EXPORT say_hello();
private:
class Impl;
std::unique_ptr<Impl> pImpl;
};
} // namespace ex
#endif
In ./src/world_priv.h:
#ifndef WORLD_PRIV_H
#define WORLD_PRIV_H
#include "world.h"
namespace ex {
class World::Impl {
public:
Impl(std::string name) : name(std::move(name)) {}
void say_hello();
void hidden();
private:
std::string name;
};
} // namespace ex
#endif
In ./src/world.cpp:
#include <iostream>
#include "world_priv.h"
namespace ex {
World::World(std::string name)
: pImpl(std::make_unique<Impl>(std::move(name))) {}
World::~World() = default;
void World::say_hello() { pImpl->say_hello(); }
void World::Impl::say_hello() {
std::cout << "Hello, World from " << name << "\n";
}
void World::Impl::hidden() { std::cout << "Not exported" << std::endl; }
} // namespace ex
In main.cpp:
#include <world.h>
int main() {
ex::World w("Earth");
w.say_hello();
}
Finally, here's the build:
cmake_minimum_required(VERSION 3.21)
project(pimpl_example)
option(BUILD_SHARED_LIBS "Build world as shared rather than static" ON)
# Library
include(GenerateExportHeader)
set(CMAKE_CXX_VISIBILITY_PRESET hidden)
set(CMAKE_VISIBILITY_INLINES_HIDDEN 1)
add_library(world src/world.cpp src/world_priv.h include/world.h)
add_library(world::world ALIAS world)
target_include_directories(
world PRIVATE "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/src>"
PUBLIC "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>"
)
generate_export_header(world EXPORT_FILE_NAME include/world_export.h)
target_compile_definitions(
world PUBLIC "$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:WORLD_STATIC_DEFINE>")
target_include_directories(
world PUBLIC "$<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/include>")
# Application
add_executable(app main.cpp)
target_link_libraries(app PRIVATE world::world)
This doesn't include install rules or anything, but it's ready for those to be written.
Building it:
$ cmake -G Ninja -S . -B build -DCMAKE_BUILD_TYPE=RelWithDebInfo
...
$ cmake --build build
$ ./build/app
Hello, World from Earth
$ $ nm ./build/libworld.so | c++filt | grep ' T ' | uniq
0000000000001460 T ex::World::say_hello()
00000000000012e0 T ex::World::World(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)
00000000000013c0 T ex::World::~World()
You can see that only the API of ex::World is exported by the library. All the private details are hidden both in the code and in the library itself.
On Windows:
>dumpbin /EXPORTS build\world.dll
Microsoft (R) COFF/PE Dumper Version 14.28.29915.0
Copyright (C) Microsoft Corporation. All rights reserved.
Dump of file build\world.dll
File Type: DLL
Section contains the following exports for world.dll
00000000 characteristics
FFFFFFFF time date stamp
0.00 version
1 ordinal base
3 number of functions
3 number of names
ordinal hint RVA name
1 0 00001390 ??0World#ex##QEAA#V?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std###Z
2 1 00001550 ??1World#ex##QEAA#XZ
3 2 000015D0 ?say_hello#World#ex##QEAAXXZ
Summary
1000 .data
1000 .pdata
2000 .rdata
1000 .reloc
1000 .rsrc
2000 .text

Answer 2

Normally, a simple way to tackle that would be to create separate public and private headers ahead of time, and only expose the public ones to the user.
Here's a simple project structure that would accomplish that:
- lib_a
- include
- main.hpp
- src
- main_private.hpp
- main.cpp
Now, obviously, that won't work for the code you posted, since the declarations you want to separate belong to the same class. But that's just a symptom of the fact that what you are trying to do is unfortunately not allowed.
From the standard basic.def.odr:
There can be more than one definition of a
(13.1) class type ([class]),
[...]
in a program provided that each definition appears in a different translation unit and the definitions satisfy the following requirements.
[...]
Each such definition shall consist of the same sequence of tokens [...]
In other words, if you put a class in a public header, it has to be identical to the one that was used when compiling the library.
As much as it would be convenient, putting "half-a-class" in a public header is just not allowed.