I am trying to make a fold expression in a function which populates the outgoing parameters of a function with some values that come in from a string vector. My fold expression is like:
((if constexpr (std::is_integral_v<Args>)
{
args = std::stoi(vec[index++]);
}
else if constexpr (std::is_same_v<Args, std::string>)
{
args = vec[index++];
}
else
{
throw std::invalid_argument("Unsupported argument type.");
}), ...);
but it fails to compile with a strange error message:
clang: error: expected expression
or
gcc: error: expected primary-expression before 'if'
(as seen on https://gcc.godbolt.org/z/xeq3j6oE7)
Does anyone have a hint on how to properly fix this?
Edit
Full context for the problem is this short application:
#include <vector>
#include <string>
#include <type_traits>
#include <iostream>
#include <stdexcept>
template <typename... Args>
void populateArgs(std::vector<std::string>& vec, Args&... args)
{
const size_t numArgs = sizeof...(Args);
if (vec.size() != numArgs)
{
throw std::invalid_argument("Number of arguments doesn't match the size of the vector.");
}
int index = 0;
((if constexpr (std::is_integral_v<Args>)
{
args = std::stoi(vec[index++]);
}
else if constexpr (std::is_same_v<Args, std::string>)
{
args = vec[index++];
}
else
{
throw std::invalid_argument("Unsupported argument type.");
}), ...);
}
int main()
{
std::vector<std::string> vec{ "1", "2", "3", "hello" };
short a;
int b;
long long c;
std::string d;
populateArgs(vec, a, b, c, d);
std::cout << "a = " << a << ", b = " << b << ", c = " << c << ", d = " << d << std::endl;
// Output: a = 1, b = 2, c = 3, d = hello
}
Like this:
([&]{
// if constexpr ...
}(), ...);
This creates a lambda and immediately calls it.
I remember this causing some problems on MSVC. If it doesn't work for you, you can try:
([&]<typename T>()
{
// Use `T` instead of `Args` here.
}.operator()<Args>(), ...);
, or a separate template function.