suppose I have the following class:
class MyInteger {
private:
int n_;
public:
MyInteger(int n) : n_(n) {};
// MORE STUFF
};
And suppose this class don't have a default trivial constructor MyInteger()
. I must always supply an int
to initialize it for some reason. And then suppose that somewhere in my code I need a vector<MyInteger>
. How do I initialize each MyInteger
component in this vector<>
?
I have two situations (probably the solution is the same, but I'll state them anyway), a normal variable inside a function:
int main(){
vector<MyInteger> foo(10); //how do I initialize each
//MyInteger field of this vector?
doStuff(foo);
}
and as data in a class:
class MyFunClass {
private:
vector<MyInteger> myVector;
public:
MyFunClass(int size, int myIntegerValue) : myVector(size) {};
// what do I put here if I need the
// initialization to call MyInteger(myIntegerValue) for all
// components of myVector?
};
Is it possible to do it just in the initialization list or must I write the initialization by hand in the MyFunClass(int, int) constructor?
This seems so very basic, and yet I somehow missed it inmy book and can't find in the web.
There are many ways to get there. Here are some of them (in no particular order of presence).
Use vector(size_type n, const T& t)
constructor. It initializes vector with n
copies of t
. For example:
#include <vector>
struct MyInt
{
int value;
MyInt (int value) : value (value) {}
};
struct MyStuff
{
std::vector<MyInt> values;
MyStuff () : values (10, MyInt (20))
{
}
};
Push elements into vector one by one. This might be useful when values should be different. For example:
#include <vector>
struct MyInt
{
int value;
MyInt (int value) : value (value) {}
};
struct MyStuff
{
std::vector<MyInt> values;
MyStuff () : values ()
{
values.reserve (10); // Reserve memory not to allocate it 10 times...
for (int i = 0; i < 10; ++i)
{
values.push_back (MyInt (i));
}
}
};
Another option is constructor initialization list, if C++0x is an option:
#include <vector>
struct MyInt
{
int value;
MyInt (int value) : value (value) {}
};
struct MyStuff
{
std::vector<MyInt> values;
MyStuff () : values ({ MyInt (1), MyInt (2), MyInt (3) /* ... */})
{
}
};
Of course, there is an option to provide default constructor and/or use something other than std::vector
.
Hope it helps.