Raise compile-time error if a string has whitespace

Question

I have a base class that is intended to be inherited by other users of the code I'm writing, and one of the abstract functions returns a name for the object. Due to the nature of the project that name cannot contain whitespace.

class MyBaseClass {

  public:

    // Return a name for this object. This should not include whitespace.
    virtual const char* Name() = 0;

};

Is there a way to check at compile-time if the result of the Name() function contains whitespace? I know compile-time operations are possible with constexpr functions but I'm not sure of the right way to signal to code users that their function returns a naughty string.

I'm also unclear on how to get a constexpr function to actually be executed by the compiler to perform such a check (if constexpr is even the way to go with this).

I don't believe there's *any* way to check a string at compile time. — Mark Ransom, Jun 10 '21 at 17:39
@MarkRansom `std::isspace` is defined with `#define` directives so it's available in `constexpr` contexts. — Michael Hoffmann, Jun 10 '21 at 17:56
That checks a single character, not a string. I stand by my statement until proven otherwise. — Mark Ransom, Jun 10 '21 at 17:59
I'm using `const char*`, not `std::string`, and for loops and array indexing are also available in `constexpr`... Hence my issue is more about checking the return value of a virtual function, not just a string (`const char*`) in general. — Michael Hoffmann, Jun 10 '21 at 18:00
Do you have a way to prevent a derived class from reading its name from a file (at program start-up)? — JaMiT, Jun 10 '21 at 18:50
@JaMiT declaring the pure virtual function as `constexpr` as in my selected answer accomplishes this. Unless there's a way to read files in a `constexpr` context. Which wouldn't even surprise me at this point. — Michael Hoffmann, Jun 10 '21 at 19:23
@MichaelHoffmann So this shouldn't surprise you: [`std::embed`](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1040r0.html#design.goals) — ph3rin, Jun 11 '21 at 00:35
Wow. C++ is just a super efficient version of JavaScript now isn't it? — Michael Hoffmann, Jun 11 '21 at 01:41

ph3rin · Accepted Answer · 2021-06-10T19:08:21.433

I think this is possible in C++20.

Here is my attempt:

#include <string_view>
#include <algorithm>
#include <stdexcept>

constexpr bool is_whitespace(char c) {
    // Include your whitespaces here. The example contains the characters
    // documented by https://en.cppreference.com/w/cpp/string/wide/iswspace
    constexpr char matches[] = { ' ', '\n', '\r', '\f', '\v', '\t' };
    return std::any_of(std::begin(matches), std::end(matches), [c](char c0) { return c == c0; });
}

struct no_ws {
    consteval no_ws(const char* str) : data(str) {
        std::string_view sv(str);
        if (std::any_of(sv.begin(), sv.end(), is_whitespace)) {
            throw std::logic_error("string cannot contain whitespace");
        }
    }
    const char* data;
};

class MyBaseClass {
  public:
    // Return a name for this object. This should not include whitespace.
    constexpr const char* Name() { return internal_name().data; }
  private:
    constexpr virtual no_ws internal_name() = 0;
};

class Dog : public MyBaseClass {
    constexpr no_ws internal_name() override {
        return "Dog";
    }
};

class Cat : public MyBaseClass {
    constexpr no_ws internal_name() override {
        return "Cat";
    }
};

class BadCat : public MyBaseClass {
    constexpr no_ws internal_name() override {
        return "Bad cat";
    }
};

There are several ideas at play here:

Let's use the type system as documentation as well as constraint. Therefore, let us create a class (no_ws in the above example) that represents a string without whitespaces.
For the type to enforce the constraints at compile-time, it must evaluate its constructor at compile time. So let's make the constructor consteval.
To ensure that derived classes don't break the contract, modify the virtual method to return no_ws.
If you want to keep the interface (i.e returning const char*), make the virtual method private, and call it in a public non-virtual method. The technique is explained here.

Now of course here I am only checking a finite set of whitespace characters and is locale-independent. I think it would very tricky to handle locales at compile-time, so maybe a better way (engineering-wise) would be to explicitly specify a set of ASCII characters allowed in the names (a whitelist instead of a blacklist).

The above example would not compile, since "Bad cat" contains whitespace. Commenting out the Bad cat class would allow the code to compile.

Live demo on Compiler Explorer

This is almost exactly what I need, very impressive. But is there an easy and `consteval`-friendly way to check a `string_view` for *any* whitespace, not just the space character? — Michael Hoffmann, Jun 10 '21 at 18:52
Also, for anyone else attempting to use this: some compilers may report `expression ‘’ is not a constant expression` rather than reporting the `logic_error`. But if the line is evaluated at all it means the condition to throw the exception was true and the string is invalid. This is the case for me with g++ version 11. — Michael Hoffmann, Jun 10 '21 at 19:11

score 1 · Answer 2 · answered Jun 10 '21 at 17:40

1

Unless the names themselves are all specified at compile-time, there's no way to assert they contain no whitespace characters prior to a runtime check.

answered Jun 10 '21 at 17:40

fbrereto

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What if I change the pure virtual declaration to `constexpr` so the return values must be specified at compile-time? – Michael Hoffmann Jun 10 '21 at 17:51
A `constexpr` function is not *guaranteed* to actually be called at compile-time, it only grants the compiler *permission* to call it, if it so chooses. `constexpr` functions can also be used at runtime, too. And besides, polymorphic function calls don't work at compile-time, anyway – Remy Lebeau Jun 10 '21 at 18:12
@RemyLebeau This is no longer true in C++ 20. Virtual functions can now be declared `constexpr`. https://stackoverflow.com/a/55972550/539997 – Michael Hoffmann Jun 10 '21 at 19:20

Raise compile-time error if a string has whitespace

2 Answers2