Construct vector of certain size

Question

Trying to construct a class attribute - a vector of certain size

class cTest
{
    public:
    std::vector<double> myTable(1900);
};

main()
{
    cTest test;

    return 0;
}

compiler says:

./src/main.cpp:43:33: error: expected identifier before numeric constant
   43 |     std::vector<double> myTable(1900);
  |                                     ^~~~

You cannot use parenthesis for a default member initializer. In this case `std::vector myTable = std::vector(1900);` is the correct solution — UnholySheep, Aug 06 '22 at 22:48
@prostargamer that will create a vector with a single element (of value `1900.0`) — UnholySheep, Aug 06 '22 at 22:50
You can also do `= decltype(myTable)(1900)` https://stackoverflow.com/q/48723015/817643 - just choose your flavour of weird — StoryTeller - Unslander Monica, Aug 06 '22 at 22:54
@ravenspoint, This looks strange because your declaration looks like a method in a class. — prostargamer, Aug 06 '22 at 22:55
Or initialize the vector at the member initializer list: https://godbolt.org/z/K14cGqd5n. Although, this way, you are creating a user-declared constructor. — rturrado, Aug 06 '22 at 23:05
They will probably address this issue in C++23 and if not then in C++26. It's been an annoying issue for quite a while. — digito_evo, Aug 06 '22 at 23:17
@Chris my bad, didn’t notice it’s a member and not a local — lorro, Aug 07 '22 at 11:26

Chris · Accepted Answer · 2022-08-07T04:47:50.360

This:

    std::vector<double> myTable(1900);

Looks like a member function declaration, and function declarations expect an argument list. 1900 does not satisfy that.

There are a couple of ways to solve this:

class cTest
{
    public:
    std::vector<double> myTable = std::vector<double>(1900);
};

If you want to make this a little less obnoxiously verbose:

class cTest
{
    using dbl_vec = std::vector<double>;

    public:
    dbl_vec myTable = dbl_vec(1900);
};

Or using the member initializer list in a constructor:

class cTest
{
    public:
    std::vector<double> myTable;

    cTest() : myTable(1900) { }
};

Construct vector of certain size

1 Answers1