constexpr function gets value at compile time even though my variable is not constexpr

Question

I'm trying to use https://github.com/gdelugre/literal_ipaddr which says it is a

C++17 constexpr implementation of inet_addr / inet_aton / inet_pton

When I do:

auto ipSourceAddressTest = IPAddr::inet_pton<AF_INET>("127.0.0.1");
std::cout << "ipSourceAddressTest is " << ipSourceAddressTest.s_addr << std::endl;

This works fine. I get the IP address in decimal.

However:

std::string ipv4address;
//get ipv4address from world here
const unsigned int ipMaxSize = 200;
char ip[ipMaxSize];
std::copy(ipv4address.begin(), ipv4address.end(), ip);
auto ipSourceAddress = IPAddr::inet_pton<AF_INET>(ip);
std::cout << "ipSourceAddress is " << ipSourceAddress.s_addr << std::endl;

Remember that ipSourceAddress.s_addr is uint32_t. The value I get printed is not the IP in decimal but in fact is 4294967295 which is 111...111 in binary. So I think it is getting its value at compile time rather than runtime.

If I do

constexpr auto in_addr1 = IPAddr::inet_pton<AF_INET>(ip);

then it's understandable that its value will be deduces in compile time. But I didn't use constexpr in my variable declaration. Does auto implies constexpr?

According to https://en.cppreference.com/w/cpp/language/constexpr,

A constexpr specifier used in a function or static member variable (since C++17) declaration implies inline

so why the function inet_pton is getting its value in compile time?

How do you know, that it is computed at compile time? Did you look at generated assembly? Note: Your ip array is uninitialized (because ipv4address is empty, and std::copy does nothing, due to that), hence using it, is undefined behavior. — Algirdas Preidžius
A constexpr function can be called at runtime. It's only calculated at compile-time if possible. — super
@AlgirdasPreidžius ipv4address is not empty, in the comment I am getting it from the world. — Guerlando OCs
@GuerlandoOCs "ipv4address is not empty, in the comment I am getting it from the world." Then, please provide minimal reproducible example. More on that: is ip being treated as null-terminated string (C-string) in IPAddr::inet_pton<AF_INET>(ip);? If so, it's undefined behavior, even if ipv4address is not empty, since std::copy wouldn't copy the null-terminator. — Algirdas Preidžius
@KaenbyouRin Looking at the library, it seems to be meant primarily for compile-time use and accepts only char arrays, not pointers. — walnut

walnut walnut · Accepted Answer · 2019-12-18T01:23:18

ipSourceAddress does not get its value at compile time (as-if rule nonwithstanding).

ip's value is not usable in a constant expression since it is not declared constexpr and doesn't qualify for one of the other exceptions to the rule of lvalue-to-rvalue conversion in a constant expression. Therefore IPAddr::inet_pton<AF_INET>(ip) is not a constant expression.

You can see this clearly by making ipSourceAddress constexpr (which auto does not imply).

constexpr on a variable requires initialization at compile-time and because the initializer is not a constant expression it will fail.

See godbolt.

I don't know how you come to a different conclusion.

Note however that the library does seem to require that the char array passed to it is exactly as long as the string it contains (plus null-terminator). It fails if you hand it a longer array and outputs the value you are seeing.

See godbolt.

It seems the author intends the functions to only be called directly with string literals.

constexpr function gets value at compile time even though my variable is not constexpr

1 Answers