Deduced template type does not work with `std::function`

Question

I'm trying to understand the root cause of compile error with following paragraphs.
My objective is that notify consumers per each setting when new setting value was updated.
Following is a simplified version which simulates when setting 1, 2 were updated


template <typename T>
struct Setting {
  T Get() {
    return T {};
  }
};

template <typename T>
void ConsumeSettingValue(Setting<T>& a, std::function<void(T)> consumer) {
  consumer(a.Get());
}

int main() {
  Setting<std::string> s1;
  Setting<int32_t> s2;

  ConsumeSettingValue(s1, [](std::string v){
    // Do consume
  });

  ConsumeSettingValue(s2, [](int32_t v){
    // Do consume
  });
}

I'm using C++ 17, clang-7, g++8 (devtoolset 8) and CentOS 7

Here are the complains from my compiler (clang-7, g++-8 emits exactly same error message, so I skipped)

[ 25%] Building CXX object CMakeFiles/UntitledLink.dir/main.cpp.o
/home/kdy/practice/untitled/main.cpp:196:3: error: no matching function for call to 'ConsumeSettingValue'
  ConsumeSettingValue(s1, [](std::string v){
  ^~~~~~~~~~~~~~~~~~~
/home/kdy/practice/untitled/main.cpp:188:6: note: candidate template ignored: could not match 'function<void (type-parameter-0-0)>' against '(lambda at /home/kdy/practice/untitled/main.cpp:196:27)'
void ConsumeSettingValue(Setting<T>& a, std::function<void(T)> consumer) {
     ^
/home/kdy/practice/untitled/main.cpp:200:3: error: no matching function for call to 'ConsumeSettingValue'
  ConsumeSettingValue(s2, [](int32_t v){
  ^~~~~~~~~~~~~~~~~~~
/home/kdy/practice/untitled/main.cpp:188:6: note: candidate template ignored: could not match 'function<void (type-parameter-0-0)>' against '(lambda at /home/kdy/practice/untitled/main.cpp:200:27)'
void ConsumeSettingValue(Setting<T>& a, std::function<void(T)> consumer) {
     ^

And it's working fine after add explicit types after ConsumeSettingValue, ConsumeSettingValue<std::string> and ConsumeSettingValue<int32_t>
but I think those lambdas can be deduced as std::function<void(T)> still, without any specific types.

Please shed some light on this. What's wrong with my theory? Thanks in advance.

score 0 · Answer 1 · answered Aug 24 '20 at 06:23

Found similar case : How to convert a lambda to an std::function using templates

The thing is that template does not consider conversions between types. Therefore std::function and given lambdas can be converted but that does not necessarily can be template deduced.

Deduced template type does not work with `std::function`

1 Answers1