1

I am trying to create a template that will accept a reference to a C-style array of objects as an argument:

#include <iostream>

class A
{
public:
A(){}
};

template<int N, class A& obj> struct W {};

int main()
{
A b[5]; 
W<5,b> w; 
}

But when compile the code I get error:

$ c++ -std=c++11 -g try37.cpp
try37.cpp: In function 'int main()':
try37.cpp:14:5: error: the value of 'b' is not usable in a constant expression
 W<5,b> w;
     ^
try37.cpp:13:3: note: 'b' was not declared 'constexpr'
 A b[5];
   ^
try37.cpp:14:6: error: could not convert template argument 'b' to 'A&'
 W<5,b> w;
      ^
try37.cpp:14:9: error: invalid type in declaration before ';' token
 W<5,b> w;
         ^

I tried many ways but was unable to fix the compilation issue? How to resolve the same?

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4
  • 1
    What about simply using std::array instead of raw arrays? Also note that template parameters refer to specify types, rather than objects. Commented Oct 6, 2018 at 18:28
  • I tried but it gives the same error. Can you please provide an example? Commented Oct 6, 2018 at 18:31
  • It does not resolves the compilation issue Commented Oct 6, 2018 at 18:34
  • 1
    Maybe you can tell us what you are trying to achieve (this seems to me to be an XY Problem), I don't think you need that kind of template Commented Oct 6, 2018 at 18:49

4 Answers 4

Reset to default
2

There are some problems in your code.

(1) if you want to pass a reference to an object as template parameter, you have to define it as constexpr and give it external static linkage (static is not necessary, correction from birdfreeyahoo (thanks!)), so 

constexpr A b[5]; 

int main ()
 {
   W<5,b> w; 
 }

(2) if you want a (C-style array of) constexpr A object(s) initialized with default constructor, you have to make contexpr the constructor too.

So

public:
constexpr A(){}

(3) if you want that the second template parameter for W is a reference to a constant C-style array of As, where the dimension is the first parameter, the syntax is

template <int N, A const (& obj)[N]>
struct W
 { };

So the full program become

class A
 {
   public:
      constexpr A ()
       { }
};

template <int N, A const (& obj)[N]>
struct W
 { };

constexpr A b[5]; 

int main ()
 {
   W<5, b> w; 
 }
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1 Comment

The global variable doesn't need to be static, since that only determines the linkage type, the storage duration is static even without the keyword
0

Edit : C style array is added to the answer.

if you trying to get struct W to have a container of objects you can use the vectors but. I'm still not sure why you want to do that. 

struct W
{
    template<typename Type, typename A> //constructor with a vector
    W(std::vector<Type,A> & vec)
    {
       //...
    }

    template<typename Type>
    W(int arraySize, Type & obj) //constructor with an array
    {
       //...

    }
};

int main()
{
    const int ArraySize = 5;
    A b[ArraySize];
    std::vector<A> vec;

    for(int i =0; i < 5; i++)
        vec.push_back(b[i]);

    W w(vec); //calling struct W constructor that takes a vector.
    W w2(ArraySize,b); //calling struct W constructor that takes c style array
    return 0;
}
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12 Comments

My requirement is that I need to create the template with a specific number of instances of class A
im still not sure why you want the template. you already know what you want to pass to W. its a class A type. are you trying to use Template to determine the size of the array you passing to the W? if so thats not the purpose of template arguments.
Class A is just used for an example - it can be just any other class
ah... i see that makes a huge difference
Thanks I think that would resolve the issue my side I can check the vector size - actually I am writing code for a module ensuring that a specific number of instances of an object needs to be passed.
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First of all, you will only be able to pass 1 instance of A to the template. Change it to A[n] obj Note: It will get converted to a pointer to A!

Second, the array you pass has to have static storage duration. So either make the array static or put it as global variable.

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3 Comments

Thanks - but how can I make it to accept a non-global variable?
Make it static, but then it acts like a global variable which just can be accessed locally (but it stays the same each function call).
If you want to have a real local variable, you simply can't, because the pointer needs to be a constexpr, and this it can only be if the address is known at compile-time (so static storage duration). This is how templates work.
0 
template < typename TElement, std::size_t NElement>
constexpr std::size_t sizeof_array( TElement const (&arr)[NElement] )
{
     (void)arr; // Unused, could have been anonymous
     return NElement;
}

template < typename TArray>
constexpr std::size_t sizeof_array2( const TArray& arr )
{
     return ( sizeof( arr ) / sizeof( arr[0] ) );
}

then in your code:

char arr[] = "abcdef;
sizeof_array(arr); // C-styled array
sizeof_array2(arr);
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