assigning constant value to struct members to be used in 1 class

Question

So far I've read about structs, with and without pointers. My question is about a struct in combination with a class and main. What I learn from K&R and c++ sites is that a struct can't contain values. The values I want to assign to the members will be constants. In several posts I read that I should put the struct inside the class and even inside private. The setup I use was:

class C
{
struct something {const float m = 3} s;` //actually 12 members. Compiler doesn't accept  values: "ISO C++ forbids initialization of member"

function foo(struct something s){ float m = s.m; <..do something..>}` //struct s actually used in several functions

};    

int main(){C c;}

Then I created 2 structs, and letting the 2nd assign values to the members of the first, which I find ugly. But why did that get accepted by gcc? So how can I assign the values only once in a proper way since assigning values has to be done inside a function. BTW, I'm using Ubuntu gcc-4.6.1.

Thank you for an answer.

You are confusing languages. In C++, `class` and `struct` are essentially the same. You can express exactly the same types using either. — juanchopanza, Oct 23 '13 at 08:27
Of course structs can contain values. And you shouldn't use K&R to learn C++, since it's about a different language. You might find a [C++ book](http://stackoverflow.com/questions/388242) more useful. — Mike Seymour, Oct 23 '13 at 08:38

nikolas · Answer 1 · 2013-10-23T14:04:48.053

1

The difference between struct and class in C++ is that structs default their members to public while classes default to private (same goes for inheritance). That's all there is to it.

Then, in C++03 const float m = 3; is not valid as a member declaration. You would need to declare a constructor for your struct:

struct something
{
    const float m;
    const float n;
    something() : m(3), n(42) {}
} s;

edited Oct 23 '13 at 14:04

answered Oct 23 '13 at 08:36

nikolas

8,707
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How do you do this when there are more constants? Now I get "something() cannot be overloaded". – HvNiekerk Oct 23 '13 at 13:53
@user2294285 updated my snippet for two constants, more are analogue – nikolas Oct 23 '13 at 14:05
Thank you. I've searched for this in Stroustrup 3rd ed., but as far as I can see the examples don't show this. Where can I find more of this ? – HvNiekerk Oct 23 '13 at 14:36
@user2294285 That concept is called an [initialization list](http://stackoverflow.com/questions/4589237/c-initialization-lists) (not to be confused with the C++11 feature `std::initializer_list`) and you should be able to find something on that; but I don't have access to the book so I can't say for sure – nikolas Oct 23 '13 at 14:41

score 1 · Answer 2 · answered Oct 23 '13 at 08:38

1

struct something {const float m = 3} s;` //actually 12 members. Compiler doesn't accept  values: "ISO C++ forbids initialization of member"

to fix above do

struct something {
    something (float const& f = 3) : m(f)
    const float m;
} s;

answered Oct 23 '13 at 08:38

Vishnu Kanwar

761
5
22

1

Why the const reference? – Luchian Grigore Oct 23 '13 at 08:41
The first line is valid C++, unless you are using C++03. – juanchopanza Oct 23 '13 at 09:23
Why the const ref? Because the default value 3 (which is literal) only have const ref. – Vishnu Kanwar Oct 23 '13 at 12:41
1

Don't use const references for PODs. The dereferencing may be more expensive than the copy. Compilers may optimize that, but I wouldn't rely on it. You may want to read [this](http://stackoverflow.com/questions/2139224/how-to-pass-objects-to-functions-in-c) – nikolas Oct 23 '13 at 14:07

score 0 · Answer 3 · answered Oct 23 '13 at 08:39

0

Structs AND classes in C++ suffer from this problem. You can only declare a value in a class definition if the variable is static and shared among all the instances of the class. If your variable is const I think that may be a good choice.

A good solution for non-const variables is a simple one - assign the desired values to all the variables in the default constructor of your class or struct.

e.g

struct Something
{
  float value;

  Something():value(5.0f) {};

}

answered Oct 23 '13 at 08:39

Lloyd Crawley

606
3
13

That doesn't apply anymore. – juanchopanza Oct 23 '13 at 09:24
@juanchopanza What doesn`t apply anymore? I made two points please be specific as to which point you are referring, and also please state which standard changed the rules. – Lloyd Crawley Oct 24 '13 at 01:23
1

Your first point. You can now say `struct Foo { int i = 42; };`. `i` doesn't have to be `const static`. – juanchopanza Oct 24 '13 at 05:04
That`s great to know! Thank you. Is this something that came with c++11 or did an earlier standard change that? Have I been sleeping for years? – Lloyd Crawley Oct 24 '13 at 06:57
It is new in C++11. You've only been sleeping for two years :-) To be fair, this feature took a while to implement in the most popular compilers. – juanchopanza Oct 24 '13 at 06:59
Thanks for the heads up. I really should learn some C++ 11 things, haven`t had the chance yet. However, the thought of C++ with even MORE features makes it that much more intimidating. – Lloyd Crawley Oct 24 '13 at 08:44

assigning constant value to struct members to be used in 1 class

3 Answers3