如何实例化对象的静态向量? [英] How to instantiate a static vector of object?

问题描述

我有一个类A，它具有对象的静态向量.这些对象属于B类

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;
}

在函数InstantiateVector()

In function InstantiateVector()

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

但是使用Visual Studio 2008时出现编译错误:无法解析的外部符号... 是否可以使用上述方法实例化静态向量?为了创建对象b，必须从输入文件中读取一些数据，并将其存储为b的成员变量

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

还是不可能，只有简单的静态向量才可能?我在某处读到要实例化静态向量，您必须首先定义const int a [] = {1,2,3}，然后将a []复制到向量中

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

推荐答案

您必须提供vector_of_b的定义，如下所示:

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;

请注意，您的InstantiateVector()制作了很多不必要的副本，这些副本可能(也可能没有)被优化了.

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());
}

实际上，对于这个仅默认构造B对象的简单示例，最简洁的方法是将循环全部替换为:

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);

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