emplace_back initialisation list error, when initialisation list works on standalone variable

Question

So I have been making a object pool class, it is used like so:

class MagicTrick {
public:
    MagicTrick(int magic) : _magic(magic)
    {}
    int predict() {
        return _number * _magic;
    }
private:
    int _magic;
    int _number = 2;
};
const std::size_t poolSize = 1;
ObjectPool<MagicTrick> magicTrickPool(poolSize, 5);

const int number = magicTrickPool.schedule([](MagicTrick& magicTrick){
    return magicTrick.predict();
});

Which works fine, however when the object used by the thread pool has its copy constructor deleted, e.g. an data member is an std::unique_ptr the construction of the pool fails. Internally I use a vector to store the pool:

struct ObjectAndLock {
    Object object;
    bool free;
    static bool isFree(const ObjectAndLock& objectAndLock) {
        return objectAndLock.free;
    }
};
std::vector<ObjectAndLock> objectAndLocks;

And I construct the full pool class:

template<typename Object> 
class ObjectPool {
    template<typename ...Args>
    ObjectPool(std::size_t poolSize, Args&&... objectArgs) 
        : objectAndLocks(poolSize, { {std::forward<Args>(objectArgs)...}, true}) 
{}

This constructs the vector with the 3rd overload listed here https://en.cppreference.com/w/cpp/container/vector/vector

This however copies the element in to the vector. So I change it to emplace_back to construct the objects in place in the vector like so:

template<typename Object> 
class ObjectPool {
    template<typename ...Args>
    ObjectPool(std::size_t poolSize, Args&&... objectArgs)
    {
        if(poolSize == 0){
            throw std::runtime_error("poolSize must be greater than 0");
        }
        objectAndLocks.reserve(poolSize);
        for (std::size_t i = 0; i < poolSize; i++)
        {
            objectAndLocks.emplace_back({Object{std::forward<Args>(objectArgs)...}, true});
        }
    }
}

This however errors with:

Projects\ObjectPool\public_include\ObjectPool\ObjectPool.hpp(87): error C2660: 'std::vector<object_pool::ObjectPool<MagicTrick>::ObjectAndLock,std::allocator<_Ty>>::emplace_back': function does not take 1 arguments
          with
          [
              _Ty=object_pool::ObjectPool<MagicTrick>::ObjectAndLock
          ]
  C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.23.28105\include\vector(651): note: see declaration of 'std::vector<object_pool::ObjectPool<MagicTrick>::ObjectAndLock,std::allocator<_Ty>>::emplace_back'
          with
          [
              _Ty=object_pool::ObjectPool<MagicTrick>::ObjectAndLock
          ]

However I can use an initializer list to create an object in the constructor like so, which compiles fine.

ObjectAndLock hello = { Object{std::forward<Args>(objectArgs)...}, true };

I have seen this answer, however I was unable to get it to work: emplace_back not working with std::vector<std::map<int, int>> I used the tempate for std::initializer_list as:

std::initializer_list<ObjectAndLock>

perhaps this is wrong?

So my question is how do I get emplace_back to work correctly? I can use up to c++17

Here is an example class that fails as it is noncopyable:

struct NonCopyable {
    std::unique_ptr<int> number = std::make_unique<int>(10);
    NonCopyable(const NonCopyable& other) = delete;
    NonCopyable& operator=(const NonCopyable& other) = delete;
};

For completeness here is the full class:

#ifndef OBJECTPOOL_H
#define OBJECTPOOL_H
#include <vector>
#include <functional>
#include <map>
#include <mutex>
#include <condition_variable>
#include <type_traits>
#include <algorithm>
#include <stdexcept>
#include <exception>
namespace object_pool {
    namespace internal {
        template <typename Function>
        class DeferToDestruction {
            Function _function;
        public:
            DeferToDestruction(Function function) : _function(function) {}
            ~DeferToDestruction() { _function(); }
        };
    }
    template<typename Object> 
    class ObjectPool {
    public:
        /*!
        @brief Create an object pool for 

        @param poolSize - Size of object pool, this must be atleast 1
        @param objectArgs... - Arguments to construct the objects in the pool

        Complete Example:
        @code
            class MagicTrick {
            public:
                MagicTrick(int magic) : _magic(magic)
                {}
                int predict() {
                    return _number * _magic;
                }
            private:
                int _magic;
                int _number = 2;
            };

            std::size_t poolSize = 5;
            object_pool::ObjectPool<MagicTrick> magicTrickPool(poolSize, 5);

            const int number = magicTrickPool.schedule([](MagicTrick& magicTrick){
                return magicTrick.predict();
            });
        @endcode

        Zero Argument Constructor Example:
        @code            
            struct ZeroArgs {
                int number = 2;
            };

            object_pool::ObjectPool<ZeroArgs> zeroArgsPool(1);
        @endcode

        Multiple Argument Constructor Example:
        @code
            class MultiArgs {
            public:
                MultiArgs(std::string name, int age, bool alive) {
                   _number = name.size() + age + (alive ? 5 : -5);
                }
                int predict() {
                    return _number * 2;
                }
            private:
                int _number = 2;
            };

            object_pool::ObjectPool<MultiArgs> multiArgsPool(1, "bob", 99, true);
        @endcode
        */
        template<typename ...Args>
        ObjectPool(std::size_t poolSize, Args&&... objectArgs)
        {
            if(poolSize == 0){
                throw std::runtime_error("poolSize must be greater than 0");
            }
            objectAndLocks.reserve(poolSize);
            for (std::size_t i = 0; i < poolSize; i++)
            {
                objectAndLocks.emplace_back({Object{std::forward<Args>(objectArgs)...}, true});
            }
        }

        ~ObjectPool(){ 

            std::unique_lock<std::mutex> lock(objectAndLocksMutex);            
            const auto allobjectAndLocksFree = [this]() {
                return std::all_of(std::begin(objectAndLocks), std::end(objectAndLocks), ObjectAndLock::isFree);
            };
            if(allobjectAndLocksFree()) {
                return;
            }
            conditionVariable.wait(lock, allobjectAndLocksFree);
        }

        /*!
        @brief Schedule access to the pool

        @param callback - An callable with the the argument being a reference to the class stored in the object pool.

        @return Returns return from the callback function, including void

        Simple Example:
        @code
            const int number = magicTrickPool.schedule([](MagicTrick& magicTrick){
                return magicTrick.predict();
            });
        @endcode
        */
        template<typename FunctionWithObjectAsParameter>
        auto schedule(FunctionWithObjectAsParameter&& callback)
        {
            const auto findFreeObject = [this]() {
                return std::find_if(std::begin(objectAndLocks), std::end(objectAndLocks), ObjectAndLock::isFree);
            };

            std::unique_lock<std::mutex> lock(objectAndLocksMutex);
            auto freeObject = findFreeObject();
            if(freeObject == std::end(objectAndLocks)) {
                conditionVariable.wait(lock, [this, &freeObject, &findFreeObject]{
                    freeObject = findFreeObject();
                    return freeObject != std::end(objectAndLocks);
                });
            }
            freeObject->free = false;
            lock.unlock();
            internal::DeferToDestruction freeAndUnlockAndNotify([this, &freeObject] () {
                {
                    std::scoped_lock<std::mutex> lock(objectAndLocksMutex);
                    freeObject->free = true;
                }
                conditionVariable.notify_one();
            });
            return callback(freeObject->object);
        }
    private:    
        struct ObjectAndLock {
            Object object;
            bool free;
            static bool isFree(const ObjectAndLock& objectAndLock) {
                return objectAndLock.free;
            }        
        };
        std::vector<ObjectAndLock> objectAndLocks;
        std::mutex objectAndLocksMutex;
        std::condition_variable conditionVariable;
    };
}
#endif

Michael Kenzel · Accepted Answer · 2019-12-04T12:10:26.973

If you look at the signature of emplace_back

template <class... Args>
reference emplace_back(Args&&... args);

you will find that the types of the parameters of emplace_back are deduced from the arguments you pass. A braced-init-list can be used to initialize an argument for a parameter of a certain type. But the {…} does not have a type itself and, thus, cannot be used to deduce the type of the parameter from.

What emplace_back does is simply std::forward whatever arguments you pass to it to the constructor of the element type to create an element in-place in the vector. The problem with that would be that your

struct ObjectAndLock {
    Object object;
    bool free;
    static bool isFree(const ObjectAndLock& objectAndLock) {
        return objectAndLock.free;
    }
};

doesn't even have a constructor that takes arguments (except for the implicit copy and move constructors).

What you would have to do is

objectAndLocks.emplace_back(ObjectAndLock{Object{std::forward<Args>(objectArgs)...}, true});

i.e., initialize a value of the right type for emplace_back to forward to the implicit move constructor. But that's essentially the same thing as just doing

objectAndLocks.push_back({Object{std::forward<Args>(objectArgs)...}, true});

The braced-init-list works with push_back because push_back

void push_back(const T& value);
void push_back(T&& value);

does expect a value of the element type rather than a pack of forwarding references and, thus, the {…} will end up initializing an argument of the appropriate type…

C++20 will introduce direct-initialization via T(…) for aggregates which will make it possible to simply write

objectAndLocks.emplace_back(Object{std::forward<Args>(objectArgs)...}, true);

here. Until then, I would recommend to just use push_back in this case…

Your

struct NonCopyable {
    std::unique_ptr<int> number = std::make_unique<int>(10);
    NonCopyable(const NonCopyable& other) = delete;
    NonCopyable& operator=(const NonCopyable& other) = delete;
};

will be not be copyable but also not moveable. You declared a copy constructor which means that there will be no implicitly-declared move constructor [class.copy]/8.1. The story is pretty much the same for the implicitly-declared move assignment operator [class.copy.assign]/4. You simply can't have an immoveable type as element type of a std::vector. To make NonCopyable moveable, you'll have to define a move constructor and move assignment operator:

struct NonCopyable {
    std::unique_ptr<int> number = std::make_unique<int>(10);

    NonCopyable(const NonCopyable&) = delete;
    NonCopyable(NonCopyable&&) = default;
    NonCopyable& operator=(const NonCopyable&) = delete;
    NonCopyable& operator=(NonCopyable&&) = default;
};

I've added an example noncopyable class to the question, it fails with `push_back` — Thomas Monkman, Dec 04 '19 at 11:27
@ThomasMonkman I've added an explanation for why `NonCopyable` won't work as element type of a `std::vector` and how to fix that. — Michael Kenzel, Dec 04 '19 at 11:37
Thank you, yes it was my class not having move functions, I assumed it generated them if copy functions where deleted. — Thomas Monkman, Dec 04 '19 at 17:43

emplace_back initialisation list error, when initialisation list works on standalone variable

1 Answers1