Is there a way to determine if a parameter is a literal string?

Question

My question is:

Is there a way to determine whether a parameter is a literal string or not?

template<class T>
bool IsLiteral(T arg)
{
    // How to implement?
}

or

template<class T>
struct IsLiteral { enum { value = ??? }; };

So that we can write the following code:

char* p = "Hello";

assert(IsLiteral(p)); // fail.
assert(IsLiteral("Hello")); // never fail.
assert(IsLiteral<decltype("Hello")>::value); // never fail.

I suppose you could make it fail when it doesn't satisfy `const char *&&`, but that wouldn't only succeed for literals. I have no idea why you want this functionality. — chris, Sep 16 '13 at 03:02
@xmllmx literal strings are related to C, but templates aren't. — Sergey Kalinichenko, Sep 16 '13 at 03:05
Another possible duplicate: [Can a compilation error be forced if a string argument is not a string literal?](http://stackoverflow.com/q/18563585/420683) — dyp, Sep 16 '13 at 03:27
@DyP I did not even think to look ... my those macros are ugly ... if I think about it [user define literals](http://en.cppreference.com/w/cpp/language/user_literal) could be a nice alternative depending on the use case. — Shafik Yaghmour, Sep 16 '13 at 03:31
@chris: Knowing if something was a string literal would be very useful. You would then be able to make an encapsulating `string_ref` like class for which you could guarantee that the source string would never change and would always be valid. — Benjamin Lindley, Sep 16 '13 at 03:50
Note that `char* p = "Hello";` should not compile on newer compilers since `"Hello"` is a `char const *` and thus it can't be assigned to a `char *` without a `const_cast()`. — Alexis Wilke, Sep 15 '21 at 16:22

score 0 · Answer 1 · edited Sep 15 '21 at 16:21

0

"String Literal" is a source code term. Once the code is compiled it assigns/loads it into a memory location. The code using its value doesn't care or know if it was created at runtime or compiled time.

edited Sep 15 '21 at 16:21

Alexis Wilke

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answered Sep 16 '13 at 03:04

Black Frog

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score 0 · Answer 2 · answered Sep 16 '13 at 03:05

There is no way to detect string literals. What can be detected is that argument is a fixed size array of char const:

template <std::size_t N>
bool IsLiteral(char const (&)[N]) {
    return true;
}
template <std::size_t N>
bool IsLiteral(char (&)[N]) {
    return false;
}
template <typename T>
bool IsLiteral(T) {
    return false;
}

I don't think you can distinguish between these two uses, however:

char const array[] = { 'a' };
IsLiteral(array);
IsLiteral("foo");

I knew that I've [seen this before...](http://stackoverflow.com/a/18563696/420683) ;) — dyp, Sep 16 '13 at 03:26

score 0 · Answer 3 · edited Sep 15 '21 at 16:19

Is there a way to determine whether a parameter is a literal string or not?

I don't think so, a string literal is prepocessing token and will be turned into an array of const char, so you won't be able to distinguish if such as array came from a string literal or not. If we look at the C++ draft standard section 2.14.5 String literals paragraph 8 says:

Ordinary string literals and UTF-8 string literals are also referred to as narrow string literals. A narrow string literal has type “array of n const char”, where n is the size of the string as defined below, and has static storage duration.

Is there a way to determine if a parameter is a literal string?

3 Answers3