用C ++构造对象

Question

So, I'm coming from Java, learning c++. I want to implement a program that can be called like this, provided by a test file (boots):

auto subject = anagram::anagram("diaper");
auto matches = subject.matches({"hello", "world", "zombies","pants"});
vector<string> expected;
BOOST_REQUIRE_EQUAL_COLLECTIONS(expected.begin(), expected.end(), matches.begin(), matches.end());

Do I get it right, that anagram::anagram(...) is a constructor, constructing an Object that provides the method matches(...) ? Because in my Implementation (cf. below) I get an error saying something (not in english) like "constructor can not be called directly". I think there is something I do not understand about constructors in c++.

// this is part of my implementation of anagram.h
class anagram{

public:
    anagram(const string a);
    vector<string> matches(vector<string> &list);

private:
    string a;
    bool isAnagram(string s);    
};

The only other thing I know is that anagram::anagram(...) could be a static method, with a return value that is similat to this , but that wouldn't make sense to me. So it would be nice if someone could explain this :)

Answer 1

Because you come from java you are used to see something similiar like this:

 someobject T = new someObject(argument);

---

in C++ you construct a class in a different way:

 someObject T(argument);

and tada you created an Object with name T in c++. an other way is to create a pointer to an object:

someObject *T = new someObject(argument);

which is more similar to java but now you have to delete that pointer manually. It is always better to create objects on the stack rather the heap. it avoids memory leaks.

---

To answer you question: You got it wrong. You call Constructors like i showed you earlier. you are right about how to call a static memberfunction but this does not apply for the Ctor because it can never be static nor virtual.

Answer 2

If the call anagram::anagram must work anagram is a namespace inside it a function called anagram is defined to return an object onto which you can call a method matches :

namespace anagram {
  SomeType anagram(string n) { return SomeType(n); }
};

That also could be a class name anagram inside a namespace anagram .

That couldn't be a static method as no method except ctors can use the name of the class as identifier.

Answer 3

One way it can work with syntax you have shown is if class anagram is placed in similary named namespace:

#include <string>
namespace anagram 
{
class anagram
{
  public:
    anagram(std::string s) {}
};
}

int main() 
{
    auto subject = anagram::anagram("diaper");
}

http://ideone.com/1OVhIz

Answer 4

The syntax would actually look a similar way as in Java (except for the new , of course) :

auto subject = Anagram("diaper");

But there's an easier way:

Anagram subject("diaper");
Anagram subject{"diaper"}; // C++11 and newer

Also do note that if Anagram has a default constructor, it will be constructed when you define it - It can't be null or something like in java, unless you are using * pointers.

Anagram subject;

You may also build a temporary Anagram object so you don't need to define the subject variable :

auto matches = Anagram{"diaper"}.matches({"hello", "world", "zombies","pants"});

If you want to use dynamic allocation, you could use new , but there isn't any GC to delete the value when you don't need anymore...!

Anagram* subject = new Anagram("diaper");
// ... do some stuff with subject...
delete subject; // Destroy the dynamically allocated subject and deallocate it

If you ever want to use dynamic memory allocation, I would recommend you to use smart pointers which you can find on this cpprefrence page . They have many advantages over manual memory allocation.