Simple Signal Library for C++11
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Title: ssig README Author: Christian Neumüller

ssig Simple Signal Library

ssig is a library which originated from the need for a simple signal library which is fast enough for signals which are invoked each frame in a game. Sadly, Boost.Signals was far too slow for this application, so I decided to write my own library. You may find the results of a benchmark at the end of this README.


ssig is header-only and requires Boost and a C++11 compiler.

The CMake files are provided to support CMake's find_package(), provide install rules and run the unit test.

If you just want to use the library, all you need to do is making the files in the include directory #includeable (e.g. by adding them to your compiler's include path or by simply copying them to the referencing project).


All functionality is provided through the <ssig.hpp> header in namespace ssig. Note: The ssig_template.hpp header must not be included by user code.

You may want to look into the unit test at test/test.cpp for usage examples.

class Signal<Signature>

template<typename R, typename A0, typename A1, ...>
class Signal<R(A0, A1, ...)>

This class provides means to connect callable entities and call them.

  • Connection<Signature> connect(boost::function<Signature> const& slot) connects slot to the signal, meaning it will be called on each invocation of the Signal's operator(). The returned Connection can be used to disconnect the signal.

  • R operator(A0, A1, ...) calls all connected slots in reverse order of connection (LIFO) with the given arguments.

    The return value is the one of the slot called last (i.e. connected first). For non-void return types, a SsigError is thrown when an attempt is made to invoke an empty Signal.

    It is allowed to disconnect any slots of the signal while (i.e. from a function called by the slot, not really concurrently from another thread) it or even the slot itself is invoked. What is not allowed, however, are recursive calls of the same Signal's operator() because then the previous would lead to undefined behavior. A SsigError is thrown in this case.

  • bool empty() const returns true when no slots are connected to the signal.

  • ~Signal() destructor: Disconnects all slots (i.e. making their isConnected() property false) and destroys the object.

class Connection<Signature>

template<typename R, typename A0, typename A1, ...>
class Connection<R(A0, A1, ...)>: public ConnectionBase

This class provides means to disconnect or invoke a particular connected slot of a Signal.

  • Connection() constructor: Constructs a disconnected connection.

  • Connection(Signal<Signature>& signal, boost::function<Signature> const&) constructor: Writing Connection<R(A0, A1, ...)> connection(signal, slot) is equivalent to auto connection = signal.connect(slot), i.e. this method connects slot to signal and then represents this connection.

  • ~Connection() destructor: destroys the Connection object without disconnecting the slot.

  • bool isConnected() const returns if the Connection has not been disconnected, i.e. is currently connecting a slot to a signal.

  • void disconnect() disconnects the slot. Requires isConnected().

  • R invokeSlot(A0, A1, ...) invokes the slot. Requires isConnected().

Any methods requiring isConnected() will throw a SsigError if this requirement is violated.

Copy semantics

If you copy a Connection, the slot is not connected again. This means that the slot will not be called additional times when invoking the signal and that disconnecting one copy of a Connection will disconnect all other copies. Moving a Connection, however, causes only the moved-from object to be disconnected (as you would expect).

class ScopedConnection<Signature>

template<typename R, typename A0, typename A1, ...>
class ScopedConnection<R(A0, A1, ...)>: public Connection<R(A0, A1, ...)>

ScopedConnection is a wrapper around a Connection which disconnects it in the destructor, if it is (still) connected. Note that copies are not disabled. As soon as the first copy is destroyed the slot (if (still) connected) will be disconnected. Moving, however is supported and does not cause the moved-from ScopedConnection to disconnect the slot (still making the moved-from object disconnected of course).

class ConnectionBase

This abstract template-less class provides only a virtual destructor and pure virtual disconnect() and isConnected() methods (as implemented by Connection<Signature>. It can be used to store Connections of different types.

class SsigError

class SsigError: public std::logic_error

This exception class is thrown by various functions of ssig.


  • SSIG_MAX_ARGS can be defined to a positive integer specifying the maximum number of slot arguments supported by ssig. It defaults to 5 and is subject to the limitations of Boost.Preprocessor.
  • SSIG_DEFINE_MEMBERSIGNAL(name, signature) convenience macro that must be used at class scope and defines a private Signal<signature> m_sig_##name member and a public connect_##name() which is a delegate of m_sig_##name.connect().
  • SSIG_DEFINE_STATICSIGNAL(name, signature) is similar to SSIG_DEFINE_MEMBERSIGNAL() but the public connect function is static and the private signal is provides in the form of a static function with the signature Signal<signature>& sig_##name() which contains the Signal as a local static variable.


Running the code from test/benchmark.cpp at commit ef32b04edd compiled with MSVC11 in release mode yields the following on my Intel Core i5 430M Windows 7 x64 notebook (with the arguments mentioned in the output):

Benchmark with 100000 signals and 10000 runs:
Test 1: std::vector of function pointers
 2.874066s wall, 2.854818s user + 0.000000s system = 2.854818s CPU (99.3%)
Test 2: std::vector of std::function
 5.484764s wall, 5.366434s user + 0.000000s system = 5.366434s CPU (97.8%)
Test 3: std::vector of boost::function
 6.032948s wall, 5.928038s user + 0.000000s system = 5.928038s CPU (98.3%)
Test 4: Boost.Signals
 114.299385s wall, 112.554722s user + 0.078001s system = 112.632722s CPU (98.5%)
Test 5: Boost.Signals2
 77.404713s wall, 76.596491s user + 0.000000s system = 76.596491s CPU (99.0%)
Test 6: ssig
 14.829950s wall, 14.508093s user + 0.000000s system = 14.508093s CPU (97.8%)
Test 6a: ssig+void
 14.184894s wall, 14.008890s user + 0.000000s system = 14.008890s CPU (98.8%)
Test 7: virtual function calls
 8.451679s wall, 8.424054s user + 0.000000s system = 8.424054s CPU (99.7%)
Total time (incl. preparation): 244.018221s wall, 240.615942s user + 0.093601s system = 240.709543s CPU (98.6%)

What immediately catches the eye is that Boost.Signals needs over 112 seconds, being even far slower than Boost.Signals2, which is interesting since Signals is, contrary to Signals2 not even thread safe. In the following table, the total CPU times of each test are summarized, with factors normalized to ssig and rounded to two places after the decimal point:

Test Time (s) Factor
vector of function pointers 2.854818 0.2
vector of std::function 5.366434 0.37
vector of boost::function 5.928038 0.41
Boost.Signals 112.632722 7.76
Boost.Signals2 76.596491 5.28
ssig 14.508093 1
ssig+void 14.008890 0.97
virtual functions 8.424054 0.58

ssig -- Copyright (c) Christian Neumüller 2012--2013 This file is subject to the terms of the BSD 2-Clause License. See LICENSE.txt or