namespace glm
{
struct vec2
{
vec2(double x_, double y_):x(x_), y(y_) {}
double x;
double y;
};
}
class polygon
{
public:
polygon() {}
polygon(std::vector<glm::vec2> vec): vertices(vec) {}
std::vector<glm::vec2> vertices;
};
要在 global scope 初始化一个复杂的变量有这些方法
https://en.cppreference.com/w/cpp/language/initialization
我只讲你这个例子几个我自己会用的方法
1. Default initialization
https://en.cppreference.com/w/cpp/language/default_initialization
polygon cube_global;
cube_global.vertices的大小是0
2. List initialization
https://en.cppreference.com/w/cpp/language/list_initialization
polygon cube_global { std::vector<glm::vec2> { glm::vec2{1.0, 2.0},
glm::vec2{3.0, 4.0} } };
cube_global.vertices的大小是2
这个初始化过程是先找到polygon的建构式
polygon(std::vector<glm::vec2> { glm::vec2{1.0, 2.0},
glm::vec2{3.0, 4.0} })
然后再找到std::vector<glm::vec2>的建构式
std::vector<glm::vec2>( { glm::vec2{1.0, 2.0},
glm::vec2{3.0, 4.0} } )
最后找到glm::vec2的建构式 (注：我没有去找glm的实作，所以先用我上面写的定义)
glm::vec2(double x_, double y_)
3. Direct initialization
https://en.cppreference.com/w/cpp/language/direct_initialization
polygon cube_global ( std::vector<glm::vec2>( 5, glm::vec2(1.0,1.0)) );
cube_global.vertices的大小是5
这很直观，就是呼叫什么建构式就做什么事
4. 使用函式进行Copy initialization(?)
https://en.cppreference.com/w/cpp/language/copy_initialization
static inline polygon polygon_factory()
{
std::vector<glm::vec2> vec2_vector;
for(int i=0;i<1250;i++)
{
vec2_vector.push_back(glm::vec2(i,i));
}
return polygon(vec2_vector);
}
polygon cube_global = polygon_factory();
cube_global.vertices的大小是1250
很特定的物件我会用这个方法