C ++委托构造函数

Question

Hello i am more familiar with Java than C++

test.h:

class Test
{
private:
int a,b,c;
public Test(int a, int b, int c);
}

test.c

Test::Test(int a, int b, int c)
{
this->a = a;
this->b = b;
this->c = c;
}
Test::Test(int a, int b)
{
this(a, b, 0);
}
Test::Test(int a)
{
this(a, 1)
}
Test::Test()
{
this(2)
}

1 - Do i have to type each constructor signature in test.h ?

2 - How can i write multiple definitions of constructors ?

3 - I read you can combine multiple constructors in 1 definition using default values. How is that done

Thank you

Answer 1

1. Yes

2. Type them out

3. Why not use Test(int _a, int _b = 1, int _c = 0);

and define with

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

ie supply default values and use base member initialisation?

This is also available pre C++11

Answer 2

as of c++11 one constructor may defer to another. So if you want to avoid publishing the default values in the interface you can do it this way:

// declaration
struct X
{
    X(int a, int b, int c);
    X(int a, int b);
    X(int a);
    X();

private:
    int a,b,c;
};

// definition    
X::X(int a, int b, int c)
: a(a), b(b), c(c)
{}


X::X(int a, int b)
: X(a, b, 2)
{
}


X::X(int a)
: X(a, 1, 2)
{
}

X::X()
: X(0,1,2)
{
}

Answer 3

Your constructor delegation syntax is wrong. You need to use the member initialiser list to delegate to a different constructor:

Test::Test(int a, int b)
    : Test(a, b, 0)
{
}
Test::Test(int a)
    : Test(a, 1)
{
}
Test::Test()
    : Test(2)
{
}

Answer 4

delegating constructors is only available in C++11. The syntax looks like this:

Test::Test(int a1, int b1, int c1) :
    a{a1}, b{b1}, c{c1}
{}

Test::Test(int a, int b) :
    Test(a, b, 0)
{}

Test::Test(int a) :
    Test(a, 1)
{}

Test::Test() :
    Test(2)
{}

I have also used the initialization syntax in the first constructor. It is preferable to do so, because otherwise the member object will be initialized with the default constructor and then assigned a new value.