C++: constructor initializer for arrays

Question

I'm having a brain cramp... how do I initialize an array of objects properly in C++?

non-array example:

struct Foo { Foo(int x) { /* ... */  } };

struct Bar { 
     Foo foo;

     Bar() : foo(4) {}
};

array example:

struct Foo { Foo(int x) { /* ... */  } };

struct Baz { 
     Foo foo[3];

     // ??? I know the following syntax is wrong, but what's correct?
     Baz() : foo[0](4), foo[1](5), foo[2](6) {}
};

edit: Wild & crazy workaround ideas are appreciated, but they won't help me in my case. I'm working on an embedded processor where std::vector and other STL constructs are not available, and the obvious workaround is to make a default constructor and have an explicit init() method that can be called after construction-time, so that I don't have to use initializers at all. (This is one of those cases where I've gotten spoiled by Java's final keyword + flexibility with constructors.)

Answer 1

There is no way. You need a default constructor for array members and it will be called, you can after do any initialization you want in the constructor.

Answer 2

Only the default constructor can be called when creating objects in an array.

Answer 3

Right now, you can't use the initializer list for array members. You're stuck doing it the hard way.

class Baz {
    Foo foo[3];

    Baz() {
        foo[0] = Foo(4);
        foo[1] = Foo(5);
        foo[2] = Foo(6);
    }
};

In C++0x you can write:

class Baz {
    Foo foo[3];

    Baz() : foo({4, 5, 6}) {}
};

Answer 4

In the specific case when the array is a data member of the class you can't initialize it in the current version of the language. There's no syntax for that. Either provide a default constructor for array elements or use std::vector.

A standalone array can be initialized with aggregate initializer

Foo foo[3] = { 4, 5, 6 };

but unfortunately there's no corresponding syntax for the constructor initializer list.

Answer 5

There is no array-construction syntax that ca be used in this context, at least not directly. You can accomplish what you're trying to accomplish by something along the lines of:

Bar::Bar()
{
    static const int inits [] = {4,5,6};
    static const size_t numInits = sizeof(inits)/sizeof(inits[0]);
    std::copy(&inits[0],&inits[numInits],foo);  // be careful that there are enough slots in foo
}

...but you'll need to give Foo a default constructor.

Answer 6

This seems to work, but I'm not convinced it's right:

#include <iostream>

struct Foo { int x; Foo(int x): x(x) { } };

struct Baz { 
     Foo foo[3];

    static int bar[3];
     // Hmm...
     Baz() : foo(bar) {}
};

int Baz::bar[3] = {4, 5, 6};

int main() {
    Baz z;
    std::cout << z.foo[1].x << "\n";
}

Output:

$ make arrayinit -B CXXFLAGS=-pedantic && ./arrayinit
g++ -pedantic    arrayinit.cpp   -o arrayinit
5

Caveat emptor.

Edit: nope, Comeau rejects it.

Another edit: This is kind of cheating, it just pushes the member-by-member array initialization to a different place. So it still requires Foo to have a default constructor, but if you don't have std::vector then you can implement for yourself the absolute bare minimum you need:

#include <iostream>

struct Foo { 
    int x; 
    Foo(int x): x(x) { }; 
    Foo(){}
};

// very stripped-down replacement for vector
struct Three { 
    Foo data[3]; 
    Three(int d0, int d1, int d2) {
        data[0] = d0;
        data[1] = d1;
        data[2] = d2;
    }
    Foo &operator[](int idx) { return data[idx]; }
    const Foo &operator[](int idx) const { return data[idx]; }
};

struct Baz { 
    Three foo;

    static Three bar;
    // construct foo using the copy ctor of Three with bar as parameter.
    Baz() : foo(bar) {}
    // or get rid of "bar" entirely and do this
    Baz(bool) : foo(4,5,6) {}
};

Three Baz::bar(4,5,6);

int main() {
    Baz z;
    std::cout << z.foo[1].x << "\n";
}

z.foo isn't actually an array, but it looks about as much like one as a vector does. Adding begin() and end() functions to Three is trivial.

Answer 7

Ideas from a twisted mind :

class mytwistedclass{
static std::vector<int> initVector;
mytwistedclass()
{
    //initialise with initVector[0] and then delete it :-)
}

};

now set this initVector to something u want to before u instantiate an object. Then your objects are initialized with your parameters.

Answer 8

Unfortunately there is no way to initialize array members till C++0x.

You could use a std::vector and push_back the Foo instances in the constructor body.

You could give Foo a default constructor (might be private and making Baz a friend).

You could use an array object that is copyable (boost or std::tr1) and initialize from a static array:

#include <boost/array.hpp>

struct Baz {

    boost::array<Foo, 3> foo;
    static boost::array<Foo, 3> initFoo;
    Baz() : foo(initFoo)
    {

    }
};

boost::array<Foo, 3> Baz::initFoo = { 4, 5, 6 };