In struct initialization

Question

Something baffles me about C++, I can write something like:

struct Test {
    double V[3];
}

without a problem, but when I try for e.g.:

struct Test {
    vector<double> V(3);
}

the compiler gives me an error. What is the difference between the above, I mean why can't the second one be compiled?

and, this one is even more baffling:

struct Test {
    std::vector<double> V[3];
}

The last example that at least in my mind should throw a compiler error is compiled without a problem by both GCC and Clang.

It seems like you want a fixed length array stl type. If this is correct look into std::array — OmnipotentEntity
– OmnipotentEntity, Commented Dec 15, 2014 at 17:26
I don't want a fixed length array, I was just doing some exploratory coding and thought that this is an inconsistency in the language. — JeanC
– JeanC, Commented Dec 15, 2014 at 17:39

gexicide · Accepted Answer · 2014-12-15 17:32:09Z

4

Do not confuse initialization and declaration.

double V[3];

This declares an array of three doubles. No problem. No initialization.

vector<double> V(3);

This declares a vector and tries to initialize it by calling its constructor with 3 as argument. Initialization like this is not allowed here. With C++11 you can use initialization syntax with curly braces, this is allowed:

vector<double> V{3};

However, since vector has a constructor that takes an initializer_list<double>, this will be interpreted as calling that constructor, not the one that takes a size_t value (the one you want). It will initialize the vector with a single element (3.0). You could use V{0,0,0} for what you want, though this might get ugly once you need more than 3 values.

Your final example

std::vector<double> V[3];

simply declares an array of three vectors. No initialization. No problem.

edited Dec 15, 2014 at 17:32

answered Dec 15, 2014 at 17:19

gexicide

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6 Comments

pm100 Over a year ago

why is V(3) not allowed?

Max Lybbert Over a year ago

V(3) is not allowed because it's currently in a place that wants a type and a variable name and nothing else, such as std::vector<double> V;. V(3) would be allowed in the constructor (or member initialzier list), where that variable needs to be initialized.

SirDarius Over a year ago

To clear any confusion, vector<double> V{3}; does not do what the OP wants. It will create a vector with one element which value is the double 3.0.

JeanC Over a year ago

Thanks SirDarius, I think gexicide meant: vector<double> V{0, 0, 0}; which is the equivalent of double V[3]; using std::vector.

T.C. Over a year ago

Or vector<double> V = vector<double>(3);, which will be much less ugly comparatively if you need 100 and not 3 elements. The grammar there requires a brace-or-equal-initializer because the parentheses version may cause ambiguities.

|

Sebastian Redl · Accepted Answer · 2014-12-15 17:20:51Z

2

Those are different things.

The first declares an array of three elements. The [3] is part of the type.

The second declares a vector, and then uses something that looks like initialization syntax, but isn't allowed in this particular context. The (3) is not part of the type. (Note that in C++11 or 14 you get in-class initializers that allow something similar.)

The third declares an array of three vectors. Compare the syntax to the first one: all that changes is that double is replaced by std::vector<double>.

answered Dec 15, 2014 at 17:20

Sebastian Redl

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Comments

OmnipotentEntity · Accepted Answer · 2014-12-15 17:28:40Z

1

It seems like you want a fixed length array stl type. std::vector isn't this. You're looking for std::array

You probably want std::array<double, 3>.

Please disregard if this isn't your intention.

answered Dec 15, 2014 at 17:28

OmnipotentEntity

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1 Comment

JeanC Over a year ago

Thanks, but I was simply trying to use a std::vector and not std::array.

SirDarius · Accepted Answer · 2014-12-15 17:33:15Z

1

What you want to do here is:

struct Test {
    vector<double> v;

    Test(): v(3) {}
};

which adds a constructor to your structure with an initialization list that explicitely invokes the vector's constructor with 3 as a parameter.

It is otherwise not possible to call a constructor in the context of declaring structure or class members (excepting initializer lists constructors with the specific C++11 syntax using curly braces).

More generally, fixed-size arrays should use the std::array class, as mentioned before.

edited Dec 15, 2014 at 17:33

answered Dec 15, 2014 at 17:24

SirDarius

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1 Comment

JeanC Over a year ago

Thanks, sorry that I can't upvote you yet (not enough points here).

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