c++结构的构造函数问题

有下面两个C++结构：
struct Vertex3D{

GLfloat x;
GLfloat y;
GLfloat z;
Vertex3D(GLfloat a, GLfloat b, GLfloat c){
    x = a;
    y = b;
    z = c;
}

};
struct Triangle3D{

Vertex3D v1;
Vertex3D v2;
Vertex3D v3;
Triangle3D(Vertex3D vt1, Vertex3D vt2, Vertex3D vt3){
};

};
Vertex3D的构造函数正常，Triangle3D的构造出现警告：“Constructor for 'Triangle3D'' must explicitly initialise member 'v2' which does not have default constructor”
但是我已经为Vertex3D类型结构创建了一构造函数，为什么还会出现这类错误呢？应该怎么解决？谢谢