Question

I want to use an array (of size count) of three fields – a long a, an int vector of length 9 named b and a bool c.

What is the right way to declare it?

Declaration 1:

vector <long a, vector<int> b(9), bool c> matrix(count);

Errors:

code.cpp: In function ‘int main()’:
code.cpp:89:49: error: template argument 1 is invalid
code.cpp:89:49: error: template argument 2 is invalid
code.cpp:89:57: error: invalid type in declaration before ‘(’ token

Declaration 2:

vector <long, vector<int>, bool> matrix(count, a, vector<int> b(9), c);

Errors:

code.cpp: In function ‘int main()’:
code.cpp:90:40: error: wrong number of template arguments (3, should be 2)
/usr/include/c++/4.5/bits/stl_vector.h:170:11: error: provided for ‘template<class _Tp, class _Alloc> class std::vector’
code.cpp:90:48: error: invalid type in declaration before ‘(’ token
code.cpp:90:56: error: ‘a’ was not declared in this scope
code.cpp:90:71: error: expected primary-expression before ‘b’
code.cpp:90:77: error: ‘c’ was not declared in this scope
code.cpp:90:78: error: initializer expression list treated as compound expression

I an new to STL and am not sure what is the right syntax here?

Answer 1

STL templates, usually, take only one parameter determining a type of contained data (and there is always a fixed number of parameters).

To get the expected effect you have to create a structure

struct s
{
  long a;
  std::vector<int> b;
  bool c;

  s(); // Declared default constructor, see below
};

and create a vector of objects of type s;

std::vector<s> matrix(count);

In order to initialize objects contained in a structure you have iterate over a vector and assign them manually, or declare a default contructor.

s::s()
: b(9) // after the colon, you can pass the constructor
// parameters for objects cointained in a structure.
{
  a = 0;
  c = false;
}