Could someone explain this C++ syntax?

Question

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What is this weird colon-member syntax in the constructor?

Hi, I recently came across this syntax in a C++ program. This is not passing parameters to a base class constructor as I know what that looks like and how to code it. This looks like some sort of variable initialization for the class... Here is the code:

class Particle
{
private:
  bool movable;
  float mass;
  Vec3 pos;
  Vec3 old_pos; 
  Vec3 acceleration;
  Vec3 accumulated_normal;
public:
  Particle(Vec3 pos)
  : pos(pos),
    old_pos(pos),
    acceleration(Vec3(0,0,0)),
    mass(1),
    movable(true),
    accumulated_normal(Vec3(0,0,0))
  {}

  Particle() {}

  // More unrelated code
};

score 6 · Answer 1 · answered Apr 28 '11 at 08:58

6

Initialisation lists can be used to initialise member variables as well as parents. This is the correct way of writing a constructor - initialisation like this is more efficient than doing assignment in the constructor body, and is likely semantically more correct.

answered Apr 28 '11 at 08:58

+1 It should also be noted that member variables have to be initialised in the order they are declared. – helpermethod Apr 28 '11 at 09:01
2

@Helper But not in the initialisation list, where order is not significant. – Apr 28 '11 at 09:02
Ah, sry, ofc you are correct. Another reason why to use them :-). – helpermethod Apr 28 '11 at 09:04
5

@Helper Method: Slightly unclear actually. They *are* initialized in the order they're declared, regardless of the order in which you write them in the initialization list. Thus you probably *should* write them in the order in which they're declared, for clarity. – Stuart Golodetz Apr 28 '11 at 09:04
2

@Stuart Golodetz: +1. Most compiler actually issue a warning when you don't. – ereOn Apr 28 '11 at 09:07
@Stuart This is true, and gcc actually produces an irritating warning if you don't. – Apr 28 '11 at 09:08
@ereOn: It's not always turned on by default :) But most compilers can be made to issue such a warning. – Stuart Golodetz Apr 28 '11 at 09:10
To follow on from that, I just tested, and VC++ with warning level 3 (the default) doesn't output such a warning. – Stuart Golodetz Apr 28 '11 at 09:13

score 3 · Accepted Answer · answered Apr 28 '11 at 08:58

That's the syntax for member initialization, as you surmised. Contrast this:

class C
{
private:
  int i;
public:
  C(int i_) : i(i_) {} // initialization
};

with:

class C
{
private:
  int i;
public:
  C(int i_) { i = i_; } // assignment
};

It's generally better to initialize members rather than assigning to them in the constructor body, and there are some cases where it's essential (one example would be references).

Could someone explain this C++ syntax?

2 Answers2