I have tried to look up on stackoverflow and other sources on how to do this but I have not find a slick way to do it using general compile time/metaprogramming techniques and the only thing I have found are things that are 70+ lines which seems a bit execissive for such a small thing, anybody got any ideas?
#Get Item From Struct at compile time
subtle remnant
hallow crescentBOT
stray wharf
Need to be more specific.
subtle remnant
So Im trying to create a variable at compile time that will be a specific type of variable from a struct. So I would probably use a template to require the name of the variable im trying to assign and the type just to make it more clear what variable im targetting. So I want to pretty much be able to extract a value from any struct
So it would work like auto struct_value = get_struct_value<variable_name, variable type>
@stray wharf More clear?
stray wharf
Kind of, but what you are looking for is probably a member pointer.
subtle remnant
Source ref so I can read?
subtle remnant
Yea well it will return a pointer from the struct
But the problem I have is how to make it in template
Because it needs to take in two template variables
stray wharf
Would need further elaboration then.
silent storm
;compile ```cpp
#include <iostream>
struct MyStruct {
int a{};
float b{};
double c{};
};
template <typename T, auto M>
auto get_struct_value(const T& obj) {
return obj.*M;
}
int main() {
MyStruct s{42, 42.f, 42.0};
auto val_a = get_struct_value<MyStruct, &MyStruct::a>(s);
std::cout << "a = " << val_a << "\n";
}
something like this?
glossy joltBOT
subtle remnant
template <typename T, auto M>
auto get_struct_value(const T& obj) {
return obj.*M;
}
This could work
silent storm
(can also be made compile time with lite dusting of constexpr)
subtle remnant
But it dosent check the variable
silent storm
But it dosent check the variable
Check it for what?
subtle remnant
So you should specify the type inside the template aswell
So that the pointer returned has the right type
So its just for checking to make sure that the variable has the right type
Because right now it just returns an int type
But I want to be able to specify any type to be refrenced to
@silent storm
silent storm
So you should specify the type inside the template aswell
You can add a type check to the template using requires if you need to, but the code would have to make some assumptions on what you would call the variable or soemthing otherwise it could look like this ```cpp
#include <iostream>
#include <utility>
struct MyStruct {
int a;
float b;
double c;
};
template <typename T, typename M_T, auto M>
constexpr M_T get_struct_value(const T& obj)
requires std::is_same_v<std::remove_cvref_t<decltype(std::declval<T>().*M)>, M_T>
{
return obj.*M;
}
int main() {
static constexpr MyStruct s{42, 3.14f, 2.71828};
// Make sure we get an int
static constexpr auto val_a = get_struct_value<MyStruct, int, &MyStruct::a>(s);
std::cout << "a = " << val_a << "\n";
return 0;
}
Still not sure what that would be used for.
subtle remnant
Im going to for example send images through a network
So Ill pull info from the sockaddr_in struct
Its a slicker way to do it in my opinion
Hopefully you get what I mean @stray wharf
stray wharf
Unfortunately I still don't.
subtle remnant
sockaddr_in has values needed to communicate with the network
So I would for example pull the in_addr and socket
subtle remnant
I just find it slick to refrence only the necessary things instead of parsing the whole struct
But I might make it a struct instead of a variable
But its a good way to filter through the necessary values
stray wharf
Parsing where?
stray wharf
Say I have a variable sockaddr_in address = getAddress(); how would your thing be better than just address.sin_addr?
subtle remnant
I dont like how it looks
And I might change the sin_addr value later
So creating a custom variable to store it is better
Since you are just refrencing the value of address.sin_addr in your example/conclusion
So refrencing would only work if I have a connection on one socket
But Im thinking of establishing a seperate connection to send the files
I could just make a new struct for that
But I find this method better since some things I might take from the other struct if I for example want both of them to use UTP or whatever
stray wharf
Demonstrate with some code.
subtle remnant
Ill demonstrate with some code when I have it
Like I said I dont currently have the code written
Prioritized this instead
But let me see
stray wharf
Pseudo-code is fine, I just need to have some idea of how it works.
subtle remnant
sockaddr_in serverAddress;
serverAddress.sin_family = AF_INET;
serverAddress.sin_port = htons(8080);
serverAddress.sin_addr.s_addr = INADDR_ANY;
bind(serverSocket, (struct sockaddr*)&serverAddress, sizeof(serverAddress));
template <typename T, typename M_T, auto M>
constexpr M_T get_struct_value(const T& obj)
requires std::is_same_v<std::remove_cvref_t<decltype(std::declval<T>().*M)>, M_T>
{
return obj.*M;
}
int SendImage(const char* image) {
constexpr static auto socket_ = get_struct_value<serverAddress, int, &serverAddress::sin_port>(s);
//Some way to add the socket_ to the new struct
struct Sockaddr_in_file {
socket_type;
};
}
I havent checked or compiled it
And Im in class so its pretty bad
But hopefully you sort of get the premise
Even though its error prone and unclear
@stray wharf
You can just run it through AI and correct it
If needed
stray wharf
sockaddr_in serverAddress;
serverAddress.sin_family = AF_INET;
serverAddress.sin...
That is not valid code.
serverAddress is a runtime value.