C++输出参数导致内存泄漏

Question

我正在开发一个基于 COM 的 C++ 项目，我的代码有一个带有 out 参数的函数，该函数将一个对象作为输入并将 class 的新实例分配给它。但是当我使用CRT Debugging时，我发现函数中存在内存泄漏，这是函数的代码。

bool __stdcall FluentCompositor::CreateCompositionHost(HWND hwnd, ICompositionHost** compositionHost)
{
    if (compositionHost != nullptr)
    {
        *compositionHost = reinterpret_cast<ICompositionHost*>(new CompositionHost(hwnd));
        if (compositionHost != nullptr)
        {
            return true;
        }
    }
    else
    {
        return false;
    }
    return false;
}

这个函数接受一个ICompositionHost的对象并用CompositionHost对象初始化它，我应该在哪里释放内存以避免内存泄漏。

我用ComPtr调用了函数，还是有内存泄漏

ComPtr<ICompositionHost> host;
compositor->CreateCompositionHost(hwnd,host.GetAddressOf());

完整代码：

FluentCompositor.cpp

#include "pch.h"
#include "ICompositionHost.h"
#include "IFluentCompositor.h"
#include "CompositionHost.h"
#include "FluentCompositor.h"

FluentCompositor::FluentCompositor() :ref(1)
{
}

ulong __stdcall FluentCompositor::AddRef()
{
    return (++ref);
}

ulong __stdcall FluentCompositor::Release()
{
    if (--ref == 0)
    {
        delete this;
        return 0;
    }
    return ref;
}

HRESULT __stdcall FluentCompositor::QueryInterface(REFIID iid, LPVOID* ppv)
{
    if (iid == IID_IFluentCompositor || iid == IID_IUnknown)
    {
        *ppv = (void*)this;
        AddRef();
    }
    else
    {
        *ppv = NULL;
    }
    return (*ppv == NULL) ? E_NOINTERFACE : S_OK;
}

HRESULT __stdcall CreateFluentCompositor(void** compositor)
{
    if (compositor != nullptr)
    {
        *compositor = reinterpret_cast<void*>(new FluentCompositor());
        if (compositor != nullptr)
        {
            return S_OK;
        }
    }
    else
    {
        return E_INVALIDARG;
    }
    return E_FAIL;
}

bool __stdcall FluentCompositor::CreateCompositionHost(HWND hwnd, ICompositionHost** compositionHost)
{
    if (compositionHost != nullptr)
    {
        *compositionHost = reinterpret_cast<ICompositionHost*>(new CompositionHost(hwnd));
        if (compositionHost != nullptr)
        {
            return true;
        }
    }
    else
    {
        return false;
    }
    return false;
}

MainWindow.cpp

#include "pch.h"
#include "MainWindow.h"

int __stdcall wWinMain(HINSTANCE, HINSTANCE, LPWSTR, int)
{
    _CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);
    const wchar_t className[] = L"Fluent Compositor";

    WNDCLASS wc = {
        .lpfnWndProc = WndProc,
        .hInstance = reinterpret_cast<HINSTANCE>(&__ImageBase),
        .hCursor = LoadCursor(nullptr, IDC_ARROW),
        .lpszClassName = className,
    };

    RegisterClass(&wc);

    HWND hwnd = CreateWindowEx(WS_EX_NOREDIRECTIONBITMAP, className, L"Fluent Compositor Sample", WS_OVERLAPPEDWINDOW | WS_VISIBLE, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, nullptr, nullptr, wc.hInstance, nullptr);
    if (hwnd == nullptr)
    {
        return 0;
    }

    CreateCompositionEffect(hwnd);

    MSG msg = { };
    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    _CrtDumpMemoryLeaks();
    return 0;
}

bool CreateCompositionEffect(HWND hwnd)
{
    auto fluentCompositorLib = LoadLibrary(L"FluentCompositor.dll");
    if (!fluentCompositorLib)
    {
        return false;
    }

    CreateCompositor = (CreateFluentCompositor)GetProcAddress(fluentCompositorLib, "CreateFluentCompositor");
    if (!CreateCompositor)
    {
        return false;
    }

    CreateCompositor(&compositor);
    if (compositor == nullptr)
    {
        return false;
    }

    ComPtr<ICompositionHost> host;
    compositor->CreateCompositionHost(hwnd,&host);
    return true;
}

LRESULT __stdcall WndProc(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam)
{
    switch (msg)
    {
        case WM_DESTROY:
        {
            PostQuitMessage(0);
            return 0;
        }
    }
    return DefWindowProc(hwnd, msg, wparam, lparam);
}

MainWindow.h

#pragma once
#include "IFluentCompositor.h"

using namespace Microsoft::WRL;

extern "C" IMAGE_DOS_HEADER __ImageBase;

ComPtr<IFluentCompositor> compositor;

typedef BOOL(__stdcall* CreateFluentCompositor)(IFluentCompositor** compositor);
CreateFluentCompositor CreateCompositor;

bool CreateCompositionEffect(HWND hwnd);
LRESULT __stdcall WndProc(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam);

Answer 1

您对 reinterpret_cast 的使用（以及一般的类型转换）完全是错误的。如果您的类实现了正确的接口，则无需手动转换它们（可能在 QueryInterface() 中除外），编译器将隐式执行 correct 转换你.

此外，您没有正确检查 new 的 return 值。或者，就此而言，处理 new 默认情况下在失败时抛出异常的可能性，而不是 returns nullptr。如果你想在失败时使用 nullptr，请改用 new 的 nothrow 版本。

此外，在使用 ComPtr 时，应该使用其重载的 operator& 而不是其 GetAddressOf() 方法。特别是如果 ComPtr 已经拥有一个接口。 GetAddressOf() 不会释放接口（这就是为什么有一个单独的 ReleaseAndGetAddressOf() 方法），但是 operator& 会。

试试这个：

#include <new>

HRESULT __stdcall FluentCompositor::QueryInterface(REFIID iid, LPVOID* ppv)
{
    if (!ppv) return E_POINTER;

    if (iid == IID_IFluentCompositor)
    {
        *ppv = static_cast<IFluentCompositor*>(this);
        /* alternatively:
        IFluentCompositor *comp = this;
        *ppv = comp;
        */
    }
    else if (iid == IID_IUnknown)
    {
        *ppv = static_cast<IUnknown*>(static_cast<IFluentCompositor*>(this));
        /* alternatively:
        IFluentCompositor *comp = this;
        IUnknown *unk = comp;
        *ppv = unk;
        */
    }
    else
    {
        *ppv = nullptr;
    }

    if (!*ppv)
        return E_NOINTERFACE;

    AddRef();
    return S_OK;
}

// similar for CompositionHost::QueryInterface() ...

HRESULT __stdcall CreateFluentCompositor(void** compositor)
{
    if (!compositor) return E_POINTER; // not E_INVALIDARG
    // make sure FluentCompositor has a refcount of 1 when created!
    *compositor = static_cast<IFluentCompositor*>(new(std::nothrow) FluentCompositor);
    return (*compositor) ? S_OK : E_FAIL;
}

bool __stdcall FluentCompositor::CreateCompositionHost(HWND hwnd, ICompositionHost** compositionHost)
{
    if (!compositionHost) return false;
    // make sure CompositionHost has a refcount of 1 when created!
    *compositionHost = new(std::nothrow) CompositionHost(hwnd);
    return (*compositionHost);
}

ComPtr<ICompositionHost> host;
compositor->CreateCompositionHost(hwnd, &host);

或者，考虑在创建对象时在内部使用 ComPtr，例如：

#include <new>

HRESULT __stdcall CreateFluentCompositor(void** compositor)
{
    if (!compositor) return E_POINTER; // not E_INVALIDARG
    // make sure FluentCompositor has a refcount of 0 when created,
    // as the ComPtr constructor will increment it!
    ComPtr<IFluentCompositor> obj(new(std::nothrow) FluentCompositor);
    return (obj) ? obj->QueryInterface(IID_IFluentCompositor, compositor) : E_FAIL;
}

bool __stdcall FluentCompositor::CreateCompositionHost(HWND hwnd, ICompositionHost** compositionHost)
{
    // make sure CompositionHost has a refcount of 0 when created,
    // as the ComPtr constructor will increment it!
    ComPtr<ICompositionHost> obj(new(std::nothrow) CompositionHost(hwnd));
    return ((obj) && (obj->QueryInterface(IID_ICompositionHost, reinterpret_cast<void**>(compositionHost)) == S_OK));
}

让你的对象以 0 而不是 1 的引用计数开始是个好主意，因为它们不知道它们是否将与接口指针或对象指针一起使用（如果它们甚至被使用有指针）。不要增加一个对象的引用计数，除非它实际上被分配给一个接口指针，该接口指针是 AddRef() 并且需要 Release()。

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