How to instantiate a static vector of object?

问题:

I have a class A, which has a static vector of objects. The objects are of class B

class A {
  public:
    static void InstantiateVector();
  private:
    static vector<B> vector_of_B;
}

In function InstantiateVector()

for (i=0; i < 5; i++) {
  B b = B();
  vector<B>.push_back(b);
}

But I have compilation error using visual studio 2008: unresolved external symbol... Is it possible to instantiate static vector using above method? For object b to be created, some data has to be read from input file, and stored as member variables of b

Or it is not possible, and only simple static vector is possible? I read somewhere that to instantiate static vector, you must first define a const int a[] = {1,2,3}, and then copy a[] into vector

回答1:

You have to provide the definition of vector_of_b as follows:

// A.h
class A {
  public:
    static void InstantiateVector();
  private:
    static vector<B> vector_of_B;
};

// A.cpp
// defining it fixes the unresolved external:
vector<B> A::vector_of_B;

As a side note, your InstantiateVector() makes a lot of unnecessary copies that may (or may not) be optimized away.

vector_of_B.reserve(5);  // will prevent the need to reallocate the underlying
                         // buffer if you plan on storing 5 (and only 5) objects
for (i=0; i < 5; i++) {
  vector_of_B.push_back(B());
}

In fact, for this simple example where you are just default constructing B objects, the most concise way of doing this is simply to replace the loop all together with:

// default constructs 5 B objects into the vector
vector_of_B.resize(5);

回答2:

Add the definition of the static member object to your class implementation file:

#include "A.h"

std::vector<B> A::vector_of_B;

Or rather, absorbing and obviating the initialization routine:

std::vector<B> A::vector_of_B(5, B()); // same effect as `A::InstantiateVector()`

_{Note: In C++98/03, vector_of_B(5) and vector_of_B(5, B()) are identical. In C++11 they're not.}

回答3:

You can either use a static helper or use boost::assign

1>using a small helper:

#include <boost\assign.hpp>
class B{};

class A {
 public:
    static bool m_helper;
    static bool InstantiateVector()
    {
        for (int i=0; i < 5; ++i) 
        {
            B b;
            vector_of_B.push_back(b);
        }
        return true;
    }
private:
    static vector<B> vector_of_B;
};

bool A::m_helper = InstantiateVector();//helper help initialize static vector here

2> Use boost::assign which is eaiser, one line is enough:

vector<B> A::vector_of_B = boost::assign::list_of(B())(B())(B())(B())(B());