algorithm.all_of()

pangram.h

#if !defined(PANGRAM_H)
#define PANGRAM_H
#include <string_view>

namespace pangram {
    bool is_pangram(std::string_view phrase);
}  // namespace pangram

#endif // PANGRAM_H

pangram.cpp

#include <algorithm>
#include <cctype>
#include <string>

namespace {
    const std::string abc = "abcdefghijklmnopqrstuvwxyz";
}

namespace pangram {
    bool is_pangram(std::string_view phrase){
        std::string lphrase(phrase.length(), ' ');
        std::transform(phrase.begin(), phrase.end(), lphrase.begin(), std::tolower);
        return std::all_of(abc.cbegin(), abc.cend(), [lphrase](char c){ return lphrase.find(c) != std::string::npos; });
  }
}  // namespace pangram

This approach starts by defining a const string to hold all of the lowercased letters of the English alphabet, within an unnamed namespace. The static keyword used to be used for this, but it was deprecated for this purpose in former versions (and then was later undeprecated.) Many coders still prefer using the unnamed namespace.

This is_pangram() function starts by defining a string to hold the lowercased letters. The transform() and tolower() functions are used to place the lowercased letters from the input phrase string_view into that string. If the currently installed locale is not suitable for lowercasing English characters, then std::locale::classic() can be passed to the tolower() function defined in <locale>, like so:

#include <algorithm>
#include <string>
#include <locale>

// code snipped

std::transform(phrase.begin(), phrase.end(), lphrase.begin(), [](char c){ return std::tolower(c, std::locale::classic()); });

The all_of() function is used test if all of the letters in the English alphabet string are present in the lowercased letters string. The lambda takes each character from the English alphabet string as an argument and passes it to the find() member function of the lowercased letters string. If find() does not return npos, then the character was found, and the lambda will return true, otherwise it will return false.

If all of the calls to find() return true, then all_of() will return true. If any call to find() returns false, then all_of() will return false.

FInally, the is_pangram() function returns the result of calling all_of().