[英]C++ String Unable to Read Memory
I have a class that looks like the following: 我有一堂课,看起来像下面这样:
class ModelCommand {
public:
virtual ~ModelCommand() {};
};
class FolderCommand : public ModelCommand {
public:
std::string text;
unsigned width;
bool isBackspace;
FolderCommand(bool isBackspace, unsigned width, std::string text = "") : text(text), width(width), isBackspace(isBackspace) {}
};
class CModel {
private:
string _folder;
public:
void Update(std::shared_ptr<ModelCommand> &cmd);
};
Then in my controller I have an instance of my model and update it using the new FolderCommand object I create: 然后在我的控制器中,我有一个模型实例,并使用我创建的新FolderCommand对象对其进行了更新:
shared_ptr<CModel> model;
shared_ptr<ModelCommand> cmd = dynamic_pointer_cast<ModelCommand>(make_shared<FolderCommand>(false, 20, "a"));
model->Update(cmd);
And then inside my update method of CModel I try to do the following: 然后在CModel的更新方法中,尝试执行以下操作:
void CModel::Update(std::shared_ptr<ModelCommand> &cmd) {
if (auto folderCmd = dynamic_pointer_cast<FolderCommand>(cmd)) {
if(!folderCmd->isBackspace)
// This is where _folder is unable to read memory
_folder += folderCmd->text;
else if(folderCmd->isBackspace && _folder.length() > 0)
_folder.erase(--_folder.end());
folderCmd->text = _folder;
}
}
This results in the CModel's _folder variable being "Unable to Read Memory". 这导致CModel的_folder变量为“无法读取内存”。
Can someone explain and provide a solution to this problem? 有人可以解释这个问题并提供解决方案吗?
Thanks. 谢谢。
UPDATE Added some more code for clarification UPDATE添加了更多代码以供澄清
Using the first snippet you posted, I called the Update method as below, and no "Unable to Read Memory" issue arose. 使用您发布的第一个代码片段,我如下调用了Update方法,并且没有出现“无法读取内存”的问题。
int main()
{
shared_ptr<ModelCommand> fc(new FolderCommand("somestring\\"));
CModel mod;
mod.Update(fc);
mod.print_folder();
return 0;
}
I think that would be interesting to see how the Update method is being used. 我想看看如何使用Update方法会很有趣。
Obs.: I added a public member function, print_folder, to print ou the _folder private member. 观察:我添加了一个公共成员函数print_folder来打印_folder私有成员。 Also, I'd make just a comment, however I still don't have this privilege. 另外,我只是发表评论,但是我仍然没有此权限。
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