class command

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The base class for commands.

template <typename... Outs>
struct command;

template <typename Out>
struct command<Out> {
  using out_type = Out;
  template <typename Answer> using resumption_type = Answer(Out);
};

template <>
struct command<> {
  using out_type = void;
  template <typename Answer> using resumption_type = Answer();
};

A class derived from command can be used as a command if it is at least copy-constructible.

Type parameters:

• typename... Outs - The return type of invoking the (derived) command (i.e., the return type of invoke_command). Should be at least move-constructible and move-assignable. Leave empty if the command should not return any value (in such case, the return type of invoke_command is void).

:large_orange_diamond: out_type

using out_type = Out;   // in command<Out>
using out_type = void;  // in command<>

The type returned by invoke_command called with the (drived) command.

:large_orange_diamond: resumption_type

template <typename Answer> using resumption_type = Answer(Out);  // in command<Out>
template <typename Answer> using resumption_type = Answer();     // in command<>

The type parameter of the resumption that captures a computation suspended by invoking the (derived) command. The type of the resumption depends also on the overall answer type of the handler, hence resumption_type is a template.

Compare the type of handle_command for a command Cmd:

Answer handle_command(Cmd, resumption<typename Cmd::template resumption_type<Answer>>)

For example, for Cmd : command<int>, the type of the corresponding handle_command in a class MyHandler : handler<bool, ..., Cmd, ...> is

bool MyHandler::handle_command(Cmd, resumption<bool(int)>)