使用初始化列表时检查传递给构造函数的参数-C ++

Question

I have a class containing vectors as data members.

When calling the constructor, I wish for said data members to be initialized by the copy constructor and not default initialized (to empty vector object), therefore I use an initialization list.

#include <vector>
using namespace std;

struct MyStruct {
    vector<double> V;
    vector<double> A;
    vector<double> B;

    MyStruct (vector<double> vee, vector< vector <double> > mat);
};

MyStruct::MyStruct (vector<double> vee, vector< vector <double> > mat)
    : V(vee),
    A(mat[0]),
    B(mat[1])    
{
    /* Rest of the constructor here*/ 
}

My questions are:

• Does doing this improves efficiency vs. permitting the default initialization and the using other methods? eg doing

  V.assign(vee)

• Is there a way to have checks and errors/messages be returned by the constructor before either default-initialization or list-initialization takes place?

I would like for the ctor to eg halt if the size of the vector<vector<double>> > 2, for the obvious reason that otherwise the above would produce a seg fault and I would like this process to be automated when the constructor is called.

Answer 1

I suppose if you want to avoid an exception you could do something like this. It uses the ternary operator to check the size of the vector and use an empty vector to initialize if one is not present in mat .

#include <vector>
using namespace std;

struct MyStruct {
    vector<double> V;
    vector<double> A;
    vector<double> B;

    MyStruct (vector<double> vee, vector< vector <double> > mat);
};

MyStruct::MyStruct (vector<double> vee, vector< vector <double> > mat)
    : V(vee),
    A(mat.size() < 1 ? vector<double>():mat[0]),
    B(mat.size() < 2 ? vector<double>():mat[1])
{
    // If you then want to throw your own exception:
    if(mat.size() < 2)
        throw std::range_error("accessing mat");

    // alternatively set a flag
    if(mat.size() < 2)
        this->good = false;
}