c-resources/CPlusPlus20ForProgrammers-m.../examples/ch15/fig15_18.cpp

40 lines
1.8 KiB
C++

// fig15_18.cpp
// Constraining a variadic-function-template parameter pack to
// elements of the same type.
#include <concepts>
#include <iostream>
#include <string>
template <typename T, typename... Us>
concept SameTypeElements = (std::same_as<T, Us> && ...);
// add one or more arguments with a fold expression
template <typename FirstItem, typename... RemainingItems>
requires SameTypeElements<FirstItem, RemainingItems...>
auto foldingSum(FirstItem first, RemainingItems... theRest) {
return (first + ... + theRest); // expand the parameter
}
int main() {
using namespace std::literals;
foldingSum(1, 2, 3); // valid: all are int values
foldingSum("s"s, "u"s, "m"s); // valid: all are std::string objects
foldingSum(1, 2.2, "hello"s); // error: three different types
}
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