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I'm not sure how the following code works. I thought you had to do {'h', 'e' ...etc...} but it seems to work fine. On the other hand if you do std::array<const char* it only adds one element to the array. Are there special rules for string literal initialization?

std::array<char, strlen("hello world!") + 1> s = {"hello world!"};
for (size_t i = 0; i < s.size(); ++i)
{
    std::cout << s[i];
}
user4051846
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2 Answers2

8

Class std::array is an aggregate. In this statement:

std::array<char, strlen("hello world!") + 1> s = {"hello world!"};

list initialization is used. As the first and only element of this instantiation of the class std::array is a character array it may be initialized with string literals.

It would be more correctly to use sizeof operator instead of function strlen:

std::array<char, sizeof( "hello world!" )> s = {"hello world!"};

Also you could write

std::array<char, sizeof( "hello world!" )> s = { { "hello world!" } };

because the character array in turn is an aggregate.

According to the C++ Standard

8.5.2 Character arrays [dcl.init.string]

1 An array of narrow character type (3.9.1), char16_t array, char32_t array, or wchar_t array can be initialized by a narrow string literal, char16_t string literal, char32_t string literal, or wide string literal, respectively, or by an appropriately-typed string literal enclosed in braces (2.14.5). Successive characters of the value of the string literal initialize the elements of the array.

[ Example:

char msg[] = "Syntax error on line %s\n";
Waqar
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Vlad from Moscow
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0

Here's a way of achieving this

// GOAL
std::array<char, sizeof("Hello")> myarray = {"Hello"};

ie. initializing std::array with a string literal (yes, it used a macro)

// SOLUTION
#define STD_CHAR_ARRAY_INIT(arrayname, string_literal) /*std::array*/<char, sizeof(string_literal)> arrayname = {string_literal}
std::array STD_CHAR_ARRAY_INIT(myarray, "Hello");

Here's some test code:

#include <iostream>
#include <array>

using std::cout;
using std::ostream;

template<typename T, size_t N>
std::ostream& std::operator<<(std::ostream& os, array<T, N> arr)
{
  {
    size_t cnt = 0;
    char strchar[2] = "x";
    for (const T& c : arr) {
      strchar[0] = c;
      os << "arr[" << cnt << "] = '" << (c == '\0' ? "\\0" :  strchar /*string(1, c)*/  ) << "'\n"
         << '.' << c << '.' << '\n';
      ++cnt;
    }
  }
  return os;
}


#define STD_CHAR_ARRAY_INIT(arrayname, string_literal) /*std::array*/<char, sizeof(string_literal)> arrayname = {string_literal}

int main()
{
  std::array STD_CHAR_ARRAY_INIT(myarray, "Hello");
  cout << myarray << '\n';
  return 0;
}
Mike
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