Some of Qt classes require that QApplication must be constructed first. So, I wrote some code for calling singleton constructor in an appropriate place. Here is a "pure" C++ code for this.
#include <functional>
#include <iostream>
#include <list>
#define INITIALIZABLE(NAME) \
private: \
template <typename T> \
friend struct Initializer; \
inline static NAME* m_instance = nullptr; \
static void create() { m_instance = new NAME; } \
inline static Initializer<NAME> m_initializer{};
struct InitQueue {
static std::list<std::function<void(void)>>& getList()
{
static std::list<std::function<void(void)>> s_list;
return s_list;
}
};
template <class T>
struct Initializer {
Initializer()
{
auto initializer = []() {
T::create();
};
InitQueue::getList().push_back(initializer);
}
};
void initialize()
{
for (auto func : InitQueue::getList()) {
func();
}
}
class Foo {
INITIALIZABLE(Foo)
public:
Foo()
{
m_count++;
std::cout << "Ctor was called: " << m_count << "\n";
}
private:
static inline int m_count = 0;
};
int main(int, char**)
{
initialize();
return 0;
}
It worked as I expected. BUT then I make a Foo Inherited from QObject, somehow ctor of Foo is calling twice.
Foo.cpp
#include "Foo.hpp"
Foo::Foo(QObject* parent)
: QObject(parent)
{
m_count++;
std::cout << "Ctor was called: " << m_count << "\n";
}
std::list<std::function<void(void)>>& InitQueue::getList()
{
static std::list<std::function<void(void)>> s_list;
return s_list;
}
void initialize()
{
for (auto func : InitQueue::getList()) {
func();
}
}
Foo.hpp
#pragma once
#include <functional>
#include <iostream>
#include <list>
#include <qobject.h>
#define INITIALIZABLE(NAME) \
private: \
template <typename T> \
friend struct Initializer; \
inline static NAME* m_instance = nullptr; \
static void create() { m_instance = new NAME; } \
inline static Initializer<NAME> m_initializer{};
struct InitQueue {
static std::list<std::function<void(void)>>& getList();
};
template <class T>
struct Initializer {
Initializer()
{
auto initializer = []() {
T::create();
};
InitQueue::getList().push_back(initializer);
}
};
void initialize();
class Foo : public QObject {
INITIALIZABLE(Foo)
public:
Foo(QObject* parent = nullptr);
private:
static inline int m_count = 0;
};
main.cpp
#include "Foo.hpp"
int main(int, char**)
{
initialize();
std::cin.get();
return 0;
}
CMakeLists.txt
cmake_minimum_required(VERSION 3.12)
project(DoubleCtorCall)
set(CMAKE_CXX_STANDARD 17)
#Qt specific
set(CMAKE_INCLUDE_CURRENT_DIR ON)
set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTOUIC ON)
set(CMAKE_AUTORCC ON)
#Qt
find_package(Qt5Core CONFIG REQUIRED)
set(SOURCES
Main.cpp Foo.cpp Foo.hpp)
add_executable(${PROJECT_NAME} ${SOURCES})
target_link_libraries(${PROJECT_NAME} Qt5::Core)
So. I have two questions: Why does this happened and how to avoid it (except once flags)?
Windows 10. MSVC 2017. Qt 5.12.
