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If I don't define a constructor in a struct, I can initialize it by just picking a certain value like this:
struct Foo {
int x, y;
};
Foo foo = {.y = 1};
But if I add new default constructor then I lose this feature:
struct Bar {
int x, y;
Bar(int value) : x(value), y(value) {}
};
Bar bar1 = 1;
Bar bar2 = {.y = 2}; // error: a designator cannot be used with a non-aggregate type "Bar"
Is it possible to have both ways?
I tried adding the default constructor Bar () {} but it seems to not work either.
You can't have your cake and eat it too. If the object has a constructor it is no longer an aggregate, and only aggregates can be initialized with designated initializers. You can't use constructors for arbitrary initialization logic with aggregates.
Are we toasted though? No, because there's the "named constructor" idiom. It's essentially just a static member function that returns an initialized object, and able to perform some logic. The idiom is compatible with aggregate initialization.
struct Foo {
int x, y;
static Foo filled_with(int value) {
return {.x = value, .y = value};
}
};
Foo foo = {.y = 1}; // Still an aggregate.
Foo foo2 = Foo::filled_with(2); // Custom logic
There's not even any copying or moving with this approach, because C++17 removed the possibility for those. foo2 is initialized directly with whatever the static member does.
Similar to what ellipticaldoor wrote:
struct FooBase {
int x = 0, y = 0;
};
struct Foo : FooBase {
Foo(int x_) : FooBase{.x = x_} { }
Foo(FooBase &&t) : FooBase{t} {}
};
Foo foo = {{.y = 1}};
Foo foo2{1};
So far this is the closest thing I can find:
struct Vec2 {
int x, y;
};
struct Bar {
int x, y;
Bar(int value) : x(value), y(value) {}
Bar(Vec2 value) : x(value.x), y(value.y){};
};
Bar bar1 = 1;
Bar bar2 = {{.y = 2}};
But you need to use double params
You can use a data member initializer instead so the type remains an aggregate:
struct Foo {
int x = 0, y = x;
};
Foo foo1 = {.y = 6}; // Foo{0, 6}
Foo foo2{7}; // Foo{7, 7}
(Though it can't be implicitly constructed from int)
Suppose I have this struct
struct MyStruct {
static MyStruct Create(int x) {
return { x*2, x>3 };
}
MyStruct(const MyStruct& c) = delete; // no copy c'tor
private:
MyStruct(int a_, bool b_) : a(a_), b(b_) {} // private c'tor -- can't use new
const int a;
const bool b;
};
Edit: I deleted the copy constructor. This is simplified example of some classes I have in my codebase where they don't have copy c'tors.
I can get an instance on the stack like so:
int main() {
auto foo = MyStruct::Create(2);
return 0;
}
But suppose I need a pointer instead (or unique_ptr is fine), and I can't change the implementation of MyStruct, how can I do that?
You could wrap MyStruct in another class, which has a MyStruct member. Here's a minimal version of that:
class Wrapper {
public:
MyStruct ms;
Wrapper(int x) : ms(MyStruct::Create(x)) { }
};
which you can use like so:
int main() {
MyStruct::Create(2);
std::make_unique<Wrapper>(2);
}
This code will not trigger any copies nor moves - because of copy elision (see: What are copy elision and return value optimization?).
You can then add any other constructors and methods you like to such a wrapper, possibly forwarding some of the method calls to the ms member. Some might choose to make ms protected or private.
Is this what you're looking for?
auto baz = std::make_unique<MyStruct>( MyStruct::Create(2) ); // unique pointer
A comment rather than an answer, to avoid confusion for future readers.
I can get an instance on the stack like so:
int main() {
auto foo = MyStruct::Create(2);
return 0;
}
Note that this is only true as of C++17 and guaranteed copy elision, whereas the program is ill-formed is C++14, as even if the copy may be elided, the initialization of foo is copy-initialization from a temporary (in C++17: the temporary is never materialized).
One more way to do it:
struct ChildStruct : public MyStruct {
ChildStruct(int x) : MyStruct(MyStruct::Create(x))
{}
};
int main() {
MyStruct *foo1 = new ChildStruct(2);
return 0;
}
C style solution. I am not sure that this is not UB, but for simple struct with 2 integer fields it should work.
int main() {
auto foo = MyStruct::Create(2);
MyStruct *p = (MyStruct*)malloc(sizeof(MyStruct));
memcpy(p, &foo, sizeof(MyStruct));
//...
free(p);
return 0;
}
I want to return a signle object from a method call which contains multiple objects created in the method.
Results calculate() {
Foo f;
Bar b;
...
Results r(f, b);
return r;
}
class Results {
private:
?
public:
Results(Foo& f, Bar& b);
Foo? getFoo();
Bar? getBar();
}
a) Should Results member variables be pointers?
private:
Foo* foo;
Bar* bar;
public:
Results(Foo& f, Bar& b) {
this->foo = &f;
this->bar = &b;
}
b) Should getFoo return Foo, Foo& or Foo*?
Use C++11's tuples(or boost's, otherwise), you're basically reimplementing them:
#include <tuple>
std::tuple<Foo, Bar> calculate() {
Foo f;
Bar b;
...
return std::make_tuple(f, b);
}
void test() {
Foo f;
Bar b;
std::tie(f, b) = calculate();
}
Note that this could be easily extended to return more than 2 objects. That's why I used std::tuple rather than std::pair as mentioned in another answer.
No, don't do it this way. Because in calculate(), Foo f and Bar b are local objects, which will go away, when you return from this function. Copy f and b into Results.
class Results {
private:
Foo foo;
Bar bar;
public:
Results(const Foo& f, const Bar& b) : foo(f), bar(b) {}
const Foo &getFoo() const { return foo; }
const Bar &getBar() const { return bar; }
}
An easier way could be to use a std::pair and return that instead
std::pair<Foo, Bar> calculate() {
Foo f;
Bar b;
...
return std::make_pair(f, b);
}
a) No. By initializing the values in calculate(), these variables "die" when the function finished executing. This way, the pointers you initialized before will point to an empty space.
b) Considering the private members not to be pointers, you can do this as you want. If you want the data to "stay" inside the object, you may take pointers or references (does not matter which one). Take "normal" variables otherwise.
a) The results member variables should be held by value. Otherwise you return the addresses (or references) of local, out-of-scope data.
b) getFoo should return a const reference to the object, or return by value, in case of POD types.
That said, consider changing your interface to accept i/o parameters of the types Foo& and Bar&, populate the variables inside and not return them. This would avoid two copies of the returned values, which are necessary otherwise.
You can also replace your Results class with a std::tuple:
std::tuple<Foo,Bar> calculate() {
Foo f;
Bar b;
...
return std::tuple(f,b);
}
// client code:
Foo foo;
Bar bar;
std::tie(foo, bar) = calculate();
Edit: If you do not use C++11 (for std::tuple) consider boost::tuple instead.
I'm still a bit uncertain what you're trying to achieve, but currently you'd run into trouble:
a and b are on the stack in calculate, you essentially return pointers to them. But as soon as calculate is finished and a and b go out of scope, you have wild pointers. That's very bad.
Create smart pointers and return them in an object if you have more return values.
Or pass pointers to a and b to calculate and create the objects on the heap with new in calculate. But be aware that you have to delete them by yourself otherwise you end up with memory leaks.
So basically, if you have more than two objects in result, then add a smart pointer like std::auto_ptr or std::shared_ptr (or if you don't use C++11 boost::shared_ptr)
I know that you can initialize structs using list syntax:
struct Foo f = {a, b, c};
return f;
Is it possible to do this in one line as you would with classes and constructors?
If you want your struct to remain a POD, use a function that creates it:
Foo make_foo(int a, int b, int c) {
Foo f = { a, b, c };
return f;
}
Foo test() {
return make_foo(1, 2, 3);
}
With C++0x uniform initialization removes the need for that function:
Foo test() {
return Foo{1, 2, 3};
// or just:
return {1, 2, 3};
}
Create a constructor for the struct (just like a class) and just do
return Foo(a,b,c);
Edit: just to clarify: structs in C++ are just like classes with the minor difference that their default access-permission is public (and not private like in a class). Therefore you can create a constructor very simply, like:
struct Foo {
int a;
Foo(int value) : a(value) {}
};
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.)
Edit: see Barry's answer for something more recent, there was no way when I answered but nowadays you are rarely limited to C++98.
There is no way. You need a default constructor for array members and it will be called, afterwards, you can do any initialization you want in the constructor.
Just to update this question for C++11, this is now both possible to do and very natural:
struct Foo { Foo(int x) { /* ... */ } };
struct Baz {
Foo foo[3];
Baz() : foo{{4}, {5}, {6}} { }
};
Those braces can also be elided for an even more concise:
struct Baz {
Foo foo[3];
Baz() : foo{4, 5, 6} { }
};
Which can easily be extended to multi-dimensional arrays too:
struct Baz {
Foo foo[3][2];
Baz() : foo{1, 2, 3, 4, 5, 6} { }
};
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}) {}
};
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 };
You can use C++0x auto keyword together with template specialization on for example a function named boost::make_array() (similar to make_pair()). For the case of where N is either 1 or 2 arguments we can then write variant A as
namespace boost
{
/*! Construct Array from #p a. */
template <typename T>
boost::array<T,1> make_array(const T & a)
{
return boost::array<T,2> ({{ a }});
}
/*! Construct Array from #p a, #p b. */
template <typename T>
boost::array<T,2> make_array(const T & a, const T & b)
{
return boost::array<T,2> ({{ a, b }});
}
}
and variant B as
namespace boost {
/*! Construct Array from #p a. */
template <typename T>
boost::array<T,1> make_array(const T & a)
{
boost::array<T,1> x;
x[0] = a;
return x;
}
/*! Construct Array from #p a, #p b. */
template <typename T>
boost::array<T,2> make_array(const T & a, const T & b)
{
boost::array<T,2> x;
x[0] = a;
x[1] = b;
return x;
}
}
GCC-4.6 with -std=gnu++0x and -O3 generates the exact same binary code for
auto x = boost::make_array(1,2);
using both A and B as it does for
boost::array<int, 2> x = {{1,2}};
For user defined types (UDT), though, variant B results in an extra copy constructor, which usually slow things down, and should therefore be avoided.
Note that boost::make_array errors when calling it with explicit char array literals as in the following case
auto x = boost::make_array("a","b");
I believe this is a good thing as const char* literals can be deceptive in their use.
Variadic templates, available in GCC since 4.5, can further be used reduce all template specialization boiler-plate code for each N into a single template definition of boost::make_array() defined as
/*! Construct Array from #p a, #p b. */
template <typename T, typename ... R>
boost::array<T,1+sizeof...(R)> make_array(T a, const R & ... b)
{
return boost::array<T,1+sizeof...(R)>({{ a, b... }});
}
This works pretty much as we expect. The first argument determines boost::array template argument T and all other arguments gets converted into T. For some cases this may undesirable, but I'm not sure how if this is possible to specify using variadic templates.
Perhaps boost::make_array() should go into the Boost Libraries?
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.
Only the default constructor can be called when creating objects in an array.
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.
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.
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.
You can do it, but it's not pretty:
#include <iostream>
class A {
int mvalue;
public:
A(int value) : mvalue(value) {}
int value() { return mvalue; }
};
class B {
// TODO: hack that respects alignment of A.. maybe C++14's alignof?
char _hack[sizeof(A[3])];
A* marr;
public:
B() : marr(reinterpret_cast<A*>(_hack)) {
new (&marr[0]) A(5);
new (&marr[1]) A(6);
new (&marr[2]) A(7);
}
A* arr() { return marr; }
};
int main(int argc, char** argv) {
B b;
A* arr = b.arr();
std::cout << arr[0].value() << " " << arr[1].value() << " " << arr[2].value() << "\n";
return 0;
}
If you put this in your code, I hope you have a VERY good reason.
This is my solution for your reference:
struct Foo
{
Foo(){}//used to make compiler happy!
Foo(int x){/*...*/}
};
struct Bar
{
Foo foo[3];
Bar()
{
//initialize foo array here:
for(int i=0;i<3;++i)
{
foo[i]=Foo(4+i);
}
}
};
in visual studio 2012 or above, you can do like this
struct Foo { Foo(int x) { /* ... */ } };
struct Baz {
Foo foo[3];
Baz() : foo() { }
};
class C
{
static const int myARRAY[10]; // only declaration !!!
public:
C(){}
}
const int C::myARRAY[10]={0,1,2,3,4,5,6,7,8,9}; // here is definition
int main(void)
{
C myObj;
}