Const data Member

[akash16]

If class have const data member, then there must be a constructor with initialiser list.

Please have a look:

class Test
{
public:
    Test();
    Test(int a, int b, int c);
    ~Test();
    void Print();
    void getConst() const;

private:
    int _a;
    int _b;
    const int _c;
};

Test::Test(int a, int b, int c): _c(c)
{
    cout<<"\nConstructor with one parameter\n";
    this->_a = a;
    this->_b = b;
}

is it correct?

The question is can we not write other than constructor with initialiser list?

please reply.

[ajh32]

Why don't you initialise all the member variables in the initialisation list:

Test::Test(int a, int b, const int c)
   : _a(a),
   , _b(b)
   , _c(c)
{

}

[Gamer2015]

If class have const data member, then there must be a constructor with initialiser list.

yeah that's correct but the initialiser list is to be prefered anyway so that's not really an issue for most of us.

[akash16]

Thanks ajh32 and Gamer2015 for reply.

@ajh32
Its just an example. We should initialise all data member(except Array type) with constructor initialiser list only, As it has its own benefits.

[MikeyBoy]

To answer your original question:

The question is can we not write other than constructor with initialiser list?

Only constructors can have initialiser lists. That includes copy constructors and move constructors.

[coder777]

From C++11 on you can intialize member directly:

class Test
{
public:
Test();
Test(int a, int b, int c);
~Test();
void Print();
void getConst() const;

private:
int _a;
int _b;
const int _c = 42; // Note
};

[andywestken]

Note that if you mix in-class initialization and initializer list, the 'list value overrides the in-class value.

{1, 2, 37}
{77, 180, 37}
{-3, -5, 37}

#include <iostream>

class Test {
public:
    Test() : _a(1), _b(2) {}
    Test(int a) : _a(a), _b(180) {}
    Test(int a, int b) : _a(a), _b(b) {}
    ~Test(){}
    void Print() const;

private:
    int _a = 17;
    const int _b = 23;
    const int _c = 37;
};

void Test::Print() const {
    std::cout << "{" << _a << ", " << _b << ", " << _c << "}\n";
};

int main() {
    Test t0;
    Test t1(77);
    Test t2(-3, -5);
    t0.Print();
    t1.Print();
    t2.Print();
    return 0;
}

[andywestken]

except Array type

C++11 also allows you to init array types.

{4, 5, 6}
{77, 2, 3}

#include <iostream>

class Test {
public:
    Test() : _a{4, 5, 6} {}
    Test(int a_0) { _a[0] = a_0; }
    ~Test(){}
    void Print() const;

private:
    int _a[3] = {1, 2, 3};
};

void Test::Print() const {
    std::cout << "{" << _a[0] << ", " << _a[1] << ", " << _a[2] << "}\n";
};

int main() {
    Test t0;
    Test t1(77);
    t0.Print();
    t1.Print();
    return 0;
}