copy-constructor

问题 I need to protect the access to a data structure in my class. As I can't have mutex (because I can't copy it) I am considering to have shared_ptr and keep the mutex there. Here is a sample code of my idea: class Sample { typedef boost::lock_guard<boost::mutex> AcquireLock; boost::shared_ptr<boost::mutex> mutt; public: Sample() : mutt(new boost::mutex) {} void Method() { AcquireLock lock(*mutt); //do some work here } }; I've got the following questions: Is it a bad practice to use the mutex

问题 Consider the following code: class A { A(const A&); public: A() {} }; int main() { const A &a = A(); } This code compiles fine with GCC 4.7.2, but fails to compile with Visual C++ 2010 with the following error: test.cc(8) : error C2248: 'A::A' : cannot access private member declared in class 'A' test.cc(2) : see declaration of 'A::A' test.cc(1) : see declaration of 'A' So is it necessary to have a copy constructor accessible when binding a temporary to a reference? This is somewhat related to

问题 Per this presentation, if either the copy constructor or copy assignment operator is "user declared", then no implicit move operations will be generated. Does delete ing the copy constructor or copy assignment operator count as "user declared"? struct NoCopy { NoCopy(NoCopy&) = delete; NoCopy& operator=(const NoCopy&) = delete; }; Will implicit move operations be generated for the NoCopy class? Or does deleting the relevant copy operations count as "user declared" and thus inhibit implicit

问题 I'm using this two classes // This is generic data structure containing some binary data class A { public: A(); A(const A&); ~A(); } // Main data container class B { public: B(); B( const B&); ~B(); protected: std::vector<A *> data; } // Copy constructor for class b B::B( const B& orig):data() { for( std::vector<A *>::const_iterator it = orig.data.begin(); it < orig.data.end(); ++it){ data.push_back( new A( *(*it))); } } I guess this class would do it job, but I'm finding to way how to reach

问题 Consider the following code: #include <iostream> using namespace std; class A { public: int a; A(): a(5) { cout << "Constructor\n"; } A(const A &b) { a = b.a; cout << "Copy Constructor\n"; } A fun(A a) { return a; } }; int main() { A a, c; A b = a.fun(c); return 0; } The output of the above code with g++ file.cpp is: Constructor Constructor Copy Constructor Copy Constructor The output of the above code with g++ -fno-elide-constructors file.cpp is: Constructor Constructor Copy Constructor Copy

问题 Can anyone explain the meaning of *p=*q in this C++ code? Is this a copy constructor concept? class A{ //any code } int main(){ A *p=new A(); A *q=new A(); *p=*q; return 0; } 回答1: Is this a copy constructor concept? No, what you are referring to is a copy assignment concept. Consider this: int* p = new int{9}; int* q = new int{10}; *p = *q; As you can see above, only the value of the variable q which is pointed to is copied. This is the equivalent of the copy assignment for objects. If you

问题 Can anyone explain the meaning of *p=*q in this C++ code? Is this a copy constructor concept? class A{ //any code } int main(){ A *p=new A(); A *q=new A(); *p=*q; return 0; } 回答1: Is this a copy constructor concept? No, what you are referring to is a copy assignment concept. Consider this: int* p = new int{9}; int* q = new int{10}; *p = *q; As you can see above, only the value of the variable q which is pointed to is copied. This is the equivalent of the copy assignment for objects. If you

问题 I implemented the copy constructor as it is described here. But still the problem is that when I update route_copy , then the same update is applied to route . So, I don't understand what is wrong in my code? public class Route implements Comparable<Route> { private List<Site> sites; public Route() { sites = new ArrayList<Site>(); } public Route(List<Site> sites) { this.sites = sites; } /** * Copy constructor */ public Route(Route r) { this(r.sites); } public void deleteSite(Site s) { this

问题 As a c++ newbe I wondered why it is usefull to explicitely 'disable' or delete the = operator and copy constructor of a class: SomeClass& operator=(SomeClass&) = delete; SomeClass(SomeClass&) = delete; I guess this makes sence if the class is a singleton. But are there any other situations? (Maybe this has something to do with performance issues?) 回答1: This has nothing to do with performance. You disallow copying whenever it does not make sense to copy your class, i.e. if it is not clear what

问题 I've read a lot about the C++ Rule of Three. Many people swear by it. But when the rule is stated, it almost always includes a word like "usually," "likely," or "probably," indicating that there are exceptions. I haven't seen much discussion of what these exceptional cases might be -- cases where the Rule of Three does not hold, or at least where adhering to it doesn't offer any advantage. My question is whether my situation is a legitimate exception to the Rule of Three. I believe that in