Posts Passing String Literals as Template Parameters in C++20
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Passing String Literals as Template Parameters in C++20

In a recent post, I described a C++20 feature that allows class literals to be used as non-type template parameters. The original feature proposal mentions how this could enable constant expression strings to be passed as template parameters as well (see P0732R2, section 3.2).

This would work by wrapping the constant expression string in a structural class literal, which would store its characters in a fixed-length array (e.g. char[N]).

The following sample demonstrates this by introducing a structural class literal wrapper type StringLiteral, which is constructed from a char array.

Note that the syntax Print<"literal"> is a syntactic sugar. The string is not being passed directly to Print, but rather undergoes an implicit conversion to our class literal StringLiteral. Note that this conversion depends on C++17’s added support of template parameter deduction using constructor arguments (the value of N can be deduced from constructor argument str).

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#include <iostream>
#include <algorithm>

/**
 * Literal class type that wraps a constant expression string.
 *
 * Uses implicit conversion to allow templates to *seemingly* accept constant strings.
 */
template<size_t N>
struct StringLiteral {
    constexpr StringLiteral(const char (&str)[N]) {
        std::copy_n(str, N, value);
    }
    
    char value[N];
};

template<StringLiteral lit>
void Print() {
    // The size of the string is available as a constant expression.
    constexpr auto size = sizeof(lit.value);

    // and so is the string's content.
    constexpr auto contents = lit.value;

    std::cout << "Size: " << size << ", Contents: " << contents << std::endl;
}

int main()
{
    Print<"literal string">(); // Prints "Size: 15, Contents: literal string"
}

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