这个C++中的内存泄漏在哪里？

Question

我被告知有几个工具,下面的代码是泄漏内存,但我们不能为我们的生活看到在哪里:

HRESULT CDatabaseValues::GetCStringField(ADODB::_RecordsetPtr& aRecordset, CString& strFieldValue,
                                         const char* strFieldName, const bool& bNullAllowed)
{
    HRESULT hr = E_FAIL;

    try
    {
        COleVariant olevar;
        olevar = aRecordset->Fields->GetItem(_bstr_t(strFieldName))->Value;
        if (olevar.vt == VT_BSTR && olevar.vt != VT_EMPTY)
        {
            strFieldValue = olevar.bstrVal;
            hr = true;
        }
        else if ((olevar.vt == VT_NULL || olevar.vt == VT_EMPTY) && bNullAllowed)
        {
            //ok, but still did not retrieve a field
            hr = S_OK;
            strFieldValue = "";
        }
    }
    catch(Exception^ error)
    {
        hr = E_FAIL;
        MLogger::Write(error);
    }
    return hr;
}

我们假设它与olevar变量有关,因为泄漏的大小与从记录集返回的字符串的大小相匹配.

我已经尝试过olevar.detach()和olevar.clear(),两者都没有效果,所以如果这是原因,我该如何释放可能在GetItem中分配的内存.如果这不是原因,那是什么？

编辑

我阅读了Ray建议的文章以及与之相关的评论,然后尝试了:

HRESULT CDatabaseValues::GetCStringField(ADODB::_RecordsetPtr& aRecordset, CString& strFieldValue,
                                         const char* strFieldName, const bool& bNullAllowed)
{
    HRESULT hr = E_FAIL;

    try
    {
        COleVariant* olevar = new COleVariant();
        _bstr_t* fieldName = new _bstr_t(strFieldName);
        *olevar = aRecordset->Fields->GetItem(*fieldName)->Value;
        if (olevar->vt == VT_BSTR && olevar->vt != VT_EMPTY)
        {
            strFieldValue = olevar->bstrVal;
            hr = true;
        }
        else if ((olevar->vt == VT_NULL || olevar->vt == VT_EMPTY) && bNullAllowed)
        {
            //ok, but still did not retrieve a field
            hr = S_OK;
            strFieldValue = "";
        }
        delete olevar;
        delete fieldName;
    }
    catch(Exception^ error)
    {
        hr = E_FAIL;
        MLogger::Write(error);
    }
    return hr;
}

olevariant和bstr的主要区别现在已明确创建和销毁.

这大约减少了泄漏量,但这里仍有一些泄漏.

解？

看看Ray关于使用Detach的建议,我想出了这个:

HRESULT CDatabaseValues::GetCStringField(ADODB::_RecordsetPtr& aRecordset, CString& strFieldValue,
                                         const char* strFieldName, const bool& bNullAllowed)
{
    HRESULT hr = E_FAIL;

    try
    {
        COleVariant olevar;
        _bstr_t fieldName = strFieldName;
        olevar = aRecordset->Fields->GetItem(fieldName)->Value;

        if (olevar.vt == VT_BSTR && olevar.vt != VT_EMPTY)
        {
            BSTR fieldValue = olevar.Detach().bstrVal;
            strFieldValue = fieldValue;
            ::SysFreeString(fieldValue);
            hr = true;
        }
        else if ((olevar.vt == VT_NULL || olevar.vt == VT_EMPTY) && bNullAllowed)
        {
            //ok, but still did not retrieve a field
            hr = S_OK;
            strFieldValue = "";
        }
        ::SysFreeString(fieldName);
    }
    catch(Exception^ error)
    {
        hr = E_FAIL;
        MLogger::Write(error);
    }
    return hr;
}

根据工具(GlowCode),这不再泄漏,但我担心在将字段分配给CString后在fieldValue上使用SysFreeString.它似乎运行,但我知道这并不表示内存损坏是免费的!

Answer 1

您必须释放为BSTR分配的内存.

见文章

哦,你必须在将VARIANT的bstr值赋给CString之前进行分离

strFieldValue = olevar.detach().bstrVal;

然后确保您的CString对象及时被正确销毁.