URL Query params builder

You are not performing any percent encoding of parameter names or values, which would likely lead to breakage or even a security vulnerability if the you are given parameters that contain a special character.

A std::map might not be the right type; a std::multimap may be more appropriate.

• Give your operators some breathing space:

  return a.first + "=" + a.second;
  

• Concatenation involves constructing and deleting many temporary strings. std::stringstream avoids it:

  std::stringstream result;
  ...
  result << '&' << a.first << '=' << a.second;
  ...
  return result.str();
  

• Pass map by reference.

There is no reason to use decltype

auto concat = [](const decltype(map)::value_type& a){
                    return a.first+"="+a.second;
                };

Why not use auto

auto concat = [](auto const& a){
                    return a.first+"="+a.second;
                };

Don't special case the '?' Just use '&' then change the one character after you have finished.

std::string to_query_params(std::map<std::string, std::string> map)
{
    if (map.empty()){
        return "";
    }
    std::string result;
    std::for_each(std::begin(map), 
                  std::end(map), 
                  [&](auto const& param){ 
                      result+="&"+concat(param); 
                  });
    // Just convert the first '&' into '?'
    result[0] = '?';
    return result;
}

The reason for this is that you use different techniques for both cases by only using one you make sure your code is DRY and consistent.

 // Version 1 parameter:
 const decltype(map)::value_type&

 // Version 2 parameter:
 decltype(map)::value_type          // different.

Also:

 // Vesion 1
 return a.first+"="+a.second;

 // Version 2:
 result+="&"+concat(param);    // concat or + "="

But rather than using a lambda we can use a range based for loop.

    std::string result;
    std::for(auto const& value: map) 
        result += "&" + concat(value); 
    );

I would probably use a std::map<std::string, std::vector<std::string>> that's because parameters in the URL allow multiple values and their order is significant.