Minimal Reproducible Example
#include <unordered_map>
#include <string>
template<class T, bool did_work>
class Test {
Test(T input) : field(input), success(did_work) {}
T field;
bool success;
};
template<typename A>
void func(std::unordered_map<std::string, Test<A, bool>> input1, A input2) {}
int main() {}
Output:
$ g++ -std=c++17 -ggdb -g3 -Wall test.cpp && ./a.out
test.cpp:12:51: error: type/value mismatch at argument 2 in template parameter list for ‘template<class T, bool did_work> class Test’
12 | void func(std::unordered_map<std::string, Test<A, bool>> input1, A input2) {}
| ^~~~
test.cpp:12:51: note: expected a constant of type ‘bool’, got ‘bool’
test.cpp:12:55: error: template argument 2 is invalid
12 | void func(std::unordered_map<std::string, Test<A, bool>> input1, A input2) {}
| ^~
test.cpp:12:55: error: template argument 5 is invalid
I need to be able to pass in unordered_map<std::string, Test<A, true>>
as well as unordered_map<std::string, Test<A, false>>
. Is there a way to do this in the function definition without changing the class Test
definition?
With template<class T, bool did_work>
did_work
is a non-type template parameter. What that means is that instead of passing a type to it, it takes a value. Since it needs a value, you make a Test
like Test<A, true>
or Test<A, false>
, not Test<A, bool>
.
For your function you can just add a non-type template parameter to do this for you like
template<typename A, bool did_work>
void func(std::unordered_map<std::string, Test<A, did_work>> input1, A input2) {}
Do it like this:
#include <unordered_map>
#include <string>
template<class T, bool did_work>
class Test {
Test(T input) : field(input), success(did_work) {}
T field;
bool success;
};
template<typename A, bool b = true>
void func(std::unordered_map<std::string, Test<A, b>> input1, A input2)
{
}
int main() {
}
I mean that you write something like:
#include <unordered_map>
#include <string>
#include <iostream>
template<class T, bool did_work>
class Test {
public:
Test(T input) : field(input), success(did_work) {}
private:
T field;
bool success;
};
template<
typename UnorderedMap
,typename A
> requires requires (UnorderedMap map){
map.size();
}
void func(
UnorderedMap input1,
A input2)
{
std::cout << input1.size();
}
int main()
{
using A = int;
Test<A, true> t{12};
std::unordered_map<std::string, Test<A, true>> map{};
func(map, 12);
//this would fail!
//func(12, 12);
}
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