Computing Length of a C String at Compile Time. Is This Really a Constexpr

Computing length of a C string at compile time. Is this really a constexpr?

Constant expressions are not guaranteed to be evaluated at compile time, we only have a non-normative quote from draft C++ standard section 5.19 Constant expressions that says this though:

[...]>[ Note: Constant expressions can be evaluated during
translation.—end note ]

You can assign the result to constexpr variable to be sure it is evaluated at compile time, we can see this from Bjarne Stroustrup's C++11 reference which says (emphasis mine):

In addition to be able to evaluate expressions at compile time, we
want to be able to require expressions to be evaluated at compile
time; constexpr in front of a variable definition does that (and
implies const):

For example:

constexpr int len1 = length("abcd") ;

Bjarne Stroustrup gives a summary of when we can assure compile time evaluation in this isocpp blog entry and says:

[...]The correct answer - as stated
by Herb - is that according to the standard a constexpr function may
be evaluated at compiler time or run time unless it is used as a
constant expression, in which case it must be evaluated at
compile-time. To guarantee compile-time evaluation, we must either use
it where a constant expression is required (e.g., as an array bound or
as a case label) or use it to initialize a constexpr. I would hope
that no self-respecting compiler would miss the optimization
opportunity to do what I originally said: "A constexpr function is
evaluated at compile time if all its arguments are constant
expressions."

So this outlines two cases where it should be evaluated at compile time:

1. Use it where a constant expression is required, this would seem to be anywhere in the draft standard where the phrase shall be ... converted constant expression or shall be ... constant expression is used, such as an array bound.
2. Use it to initialize a constexpr as I outline above.

Evaluating strlen at compilation time?

21.8, 1 and 2, in the standard says:

1. Tables 74, 75, 76, 77, 78, and 79 describe headers <cctype>, <cwctype>, <cstring>, <cwchar>, <cstdlib>
   (character conversions), and <cuchar>, respectively.

2. The contents of these headers shall be the same as the Standard C Library headers <ctype.h>, <wctype.h>,
   <string.h>, <wchar.h>, and <stdlib.h> and the C Unicode TR header <uchar.h>, respectively, with the
   following modifications:

strlen is defined in in <cstring> in c++. The modifications that follow do not mention strlen. From that, I would conclude that the signature in C++ must be exactly the same as it is in C, and since C does not have constexpr, it is technically non-compliant. That said, this is one of those non-compliant things that's unlikely to do any harm, beyond relying on it on one platform and then not finding it on another.

compile-time variable-length objects based on string

Adding a CTAD to UsbDescrString should be enough

template<size_t N>
struct UsbDescrString final: UsbDescrStd {
    char str[N * 2];

    constexpr UsbDescrString(const char (&s)[N+1]) noexcept
        : UsbDescrStd{sizeof(*this), UsbDescriptorType::USB_DESCR_STRING}
        , str {}
    {
        for(size_t i = 0; i < N; ++i)
            str[i * 2] = s[i];
    }
};

template<size_t N>
UsbDescrString(const char (&)[N]) -> UsbDescrString<N-1>;

Note that in order to prevent array to pointer decay, const char (&) needs to be used as the constructor parameter.

Demo

"String argument to template" is explicitly prohibited as of C++20,
cutting that solution off as well.

However, thanks to P0732, with the help of some helper classes such as basic_fixed_string, now in C++20 you can

template<fixed_string>
struct UsbDescrString final: UsbDescrStd;

constexpr UsbDescrString<"Descr str"> uds9;

What is preventing compile time evaluation of this constexpr function?

You have set optimisation to level 1 in Godbolt! Try -O3 instead of -O1.

Compile time string encryption using constexpr

Here's how I would do it:

1.) Use the str_const template for constexpr string manipulation described here: Conveniently Declaring Compile-Time Strings in C++

Code:

class str_const {
    // constexpr string
private:
    const char * const p_;
    const std::size_t sz_;

public:
    template <std::size_t N>
    constexpr str_const(const char(&a)[N])
            : p_(a)
            , sz_(N - 1)
    {}

    constexpr char operator[](std::size_t n) const { return n < sz_ ? p_[n] : throw std::out_of_range(""); }
    constexpr std::size_t size() const { return sz_; }

    constexpr const char * get() const { return p_; }
};

This lets you do things like str_const message = "Invalid license" and manipulate message in constexpr functions.

2.) Make a simple compile-time pseudorandom generator, using the macros __TIME__ and __LINE__ to generate the seed. This is described in detail here: Generate random numbers in C++ at compile time

They give some template-based code.

3.) Make a struct, with a constexpr ctor which takes either const char [] and templates itself against the size similarly to the str_const example, or which just takes a str_const, and generates two str_const which it are its member variables.

• A str_const of length n containing pseudorandom unsigned chars, generated using the pseudorandom generator, where n is the length of the input. (the "noise string")
• A str_const of length n containing the entry-wise sum (as unsigned chars) of the input characters with the noise characters. (the "cipher text")

Then it has a member function decrypt which need not be constexpr, and can return a std::string, which simply subtracts each character of the noise string from the corresponding character of the cipher text and returns the resulting string.

If your compiler is still storing the original string literal in the binary, it means that either it's storing the input string literal (the constructor argument) which I don't think it should be doing since its a temporary, or its basically inlining the decrypt function, and you should be able to prevent that by obfuscating it with function pointers, or marking it volatile or similar.

Edit: I'm not sure if the standard requires that temporary constexpr objects should not appear in the binary. Actually I'm curious about that now. My expectation is that at least in a release build, a good compiler should remove them when they are no longer needed.

Edit: So, you already accepted my answer. But anyways for completeness, here's some source code that implements the above ideas, using only C++11 standard. It works on gcc-4.9 and clang-3.6, even when optimizations are disabled, as nearly as I can tell.

#include <array>
#include <iostream>
#include <string>

typedef uint32_t u32;
typedef uint64_t u64;
typedef unsigned char uchar;

template<u32 S, u32 A = 16807UL, u32 C = 0UL, u32 M = (1UL<<31)-1>
struct LinearGenerator {
    static const u32 state = ((u64)S * A + C) % M;
    static const u32 value = state;
    typedef LinearGenerator<state> next;
    struct Split { // Leapfrog
        typedef LinearGenerator< state, A*A, 0, M> Gen1;
        typedef LinearGenerator<next::state, A*A, 0, M> Gen2;
    };
};

// Metafunction to get a particular index from generator
template<u32 S, std::size_t index>
struct Generate {
    static const uchar value = Generate<LinearGenerator<S>::state, index - 1>::value;
};

template<u32 S>
struct Generate<S, 0> {
    static const uchar value = static_cast<uchar> (LinearGenerator<S>::value);
};

// List of indices
template<std::size_t...>
struct StList {};

// Concatenate
template<typename TL, typename TR>
struct Concat;

template<std::size_t... SL, std::size_t... SR>
struct Concat<StList<SL...>, StList<SR...>> {
    typedef StList<SL..., SR...> type;
};

template<typename TL, typename TR>
using Concat_t = typename Concat<TL, TR>::type;

// Count from zero to n-1
template<size_t s>
struct Count {
    typedef Concat_t<typename Count<s-1>::type, StList<s-1>> type;
};

template<>
struct Count<0> {
    typedef StList<> type;
};

template<size_t s>
using Count_t = typename Count<s>::type;

// Get a scrambled character of a string
template<u32 seed, std::size_t index, std::size_t N>
constexpr uchar get_scrambled_char(const char(&a)[N]) {
    return static_cast<uchar>(a[index]) + Generate<seed, index>::value;
}

// Get a ciphertext from a plaintext string
template<u32 seed, typename T>
struct cipher_helper;

template<u32 seed, std::size_t... SL>
struct cipher_helper<seed, StList<SL...>> {
    static constexpr std::array<uchar, sizeof...(SL)> get_array(const char (&a)[sizeof...(SL)]) {
        return {{ get_scrambled_char<seed, SL>(a)... }};
    }
};

template<u32 seed, std::size_t N>
constexpr std::array<uchar, N> get_cipher_text (const char (&a)[N]) {
    return cipher_helper<seed, Count_t<N>>::get_array(a);
}

// Get a noise sequence from a seed and string length
template<u32 seed, typename T>
struct noise_helper;

template<u32 seed, std::size_t... SL>
struct noise_helper<seed, StList<SL...>> {
    static constexpr std::array<uchar, sizeof...(SL)> get_array() {
        return {{ Generate<seed, SL>::value ... }};
    }
};

template<u32 seed, std::size_t N>
constexpr std::array<uchar, N> get_key() {
    return noise_helper<seed, Count_t<N>>::get_array();
}


/*
// Get an unscrambled character of a string
template<u32 seed, std::size_t index, std::size_t N>
char get_unscrambled_char(const std::array<uchar, N> & a) {
    return static_cast<char> (a[index] - Generate<seed, index>::value);
}
*/

// Metafunction to get the size of an array
template<typename T>
struct array_info;

template <typename T, size_t N>
struct array_info<T[N]>
{
    typedef T type;
    enum { size = N };
};

template <typename T, size_t N>
struct array_info<const T(&)[N]> : array_info<T[N]> {};

// Scramble a string
template<u32 seed, std::size_t N>
class obfuscated_string {
private:
    std::array<uchar, N> cipher_text_;
    std::array<uchar, N> key_;
public:
    explicit constexpr obfuscated_string(const char(&a)[N])
        : cipher_text_(get_cipher_text<seed, N>(a))
        , key_(get_key<seed,N>())
    {}

    operator std::string() const {
        char plain_text[N];
        for (volatile std::size_t i = 0; i < N; ++i) {
            volatile char temp = static_cast<char>( cipher_text_[i] - key_[i] );
            plain_text[i] = temp;
        }
        return std::string{plain_text, plain_text + (N - 1)};///We do not copy the termination character
    }
};

template<u32 seed, std::size_t N>
std::ostream & operator<< (std::ostream & s, const obfuscated_string<seed, N> & str) {
    s << static_cast<std::string>(str);
    return s;
}

#define RNG_SEED ((__TIME__[7] - '0') * 1  + (__TIME__[6] - '0') * 10  + \
              (__TIME__[4] - '0') * 60   + (__TIME__[3] - '0') * 600 + \
              (__TIME__[1] - '0') * 3600 + (__TIME__[0] - '0') * 36000) + \
              (__LINE__ * 100000)


#define LIT(STR) \
    obfuscated_string<RNG_SEED, array_info<decltype(STR)>::size>{STR}

auto S2 = LIT(("Hewwo, I'm hunting wabbits"));

int main() {
    constexpr auto S1 = LIT(("What's up doc"));
    std::cout << S1 << std::endl;
    std::cout << S2 << std::endl;
}

---

Related Topics