编译错误：无效使用void表达式

Question

I have a function which will pop task-codes from a queue and search it in a map of task-codes and tasks(methods) and push the the matched task into another queue which I want to pop out and executed sequentially later. Below is the function:

void CertManMgmtEECertController::PrepareTask()
{
    while(_taskCodeQueue.empty())
    {
        _taskQueue.push((_taskStore.find(_taskCodeQueue.pop()))->second);
    }
}

Below is the header where the class of this function is defined:

class CertManMgmtEECertController : public virtual CertManMgmtCertificate
{
    public:
        CertManMgmtEECertController();
        ~CertManMgmtEECertController();
        void PerformTask();
        void SetTask(CertManMgmtEETaskCode taskCode);
        typedef void (CertManMgmtEECertController::*Task)();

    private:
        CertManMgmtEETaskCode _task;
        queue<Task> _taskQueue;
        queue<CertManMgmtEETaskCode> _taskCodeQueue;
        map<CertManMgmtEETaskCode,Task> _taskStore;

        void LoadTasks();
        void PrepareTask();
        void ExecuteTaskQueue();
        void GetEECert();
        bool GetCertificate();
};

But I am getting this below error while compiling:

CertManMgmtDomainController.h:49: error: 'CertManMgmtEECertController' does not name a type
CertManMgmtEECertController.cpp: In member function 'void certman::CertManMgmtEECertController::PrepareTask()':
CertManMgmtEECertController.cpp:42: error: invalid use of void expression
CertManMgmtEECertController.cpp: In member function 'void certman::CertManMgmtEECertController::ExecuteTaskQueue()':
CertManMgmtEECertController.cpp:55: error: void value not ignored as it ought to be

Where am I going wrong? I know there are other compilation errors here too. Please help.

Answer 1

In principle the question contains enough information to answer about invalid use of void expression error.

std::queue::pop() returns void . What you want in PrepareTask is probably

while (!_taskCodeQueue.empty()) {
//    ^^^ also fixed condition
    _taskQueue.push((_taskStore.find(_taskCodeQueue.front()))->second);
    _taskCodeQueue.pop();
}

Answer 2

A good rule of thumb is to start with the first error message .

In your case that is

 CertManMgmtDomainController.h:49: error: 'CertManMgmtEECertController' does not name a type 

Possibly you have a typo, possibly the header is not included in the implementation file, possibly the identifier has been redefined as a function, whatever. But start with this. Ignore the rest of the errors: they are quite probably possibly caused by this first one.

Edit: After writing the above I see in another answer that you're using a std::queue , and indeed, its pop method returns void . This is for exception safety. There is no convenience method that combines pop and front .

---

Regarding the typo possibility, note that

• modern IDEs such as Visual Studio provide tooltips about identifiers, and

• C++ provides namespaces to deal with common prefixes in a more practical way.

Answer 3

查看是否在typedef帮助中添加namespace 。

 typedef void (certman::CertManMgmtEECertController::*Task)();