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scanf for modern C++
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Fix link in README
Latest commit d98f3b6 Jun 25, 2019


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#include <scn/scn.h>
#include <cstdio>

int main() {
    int i;
    scn::prompt("What's your favorite number?", "{}", i);
    printf("Oh, cool, %d!", i);

// Example result:
// What's your favorite number? 42
// Oh, cool, 42!

What is this?

scnlib is a modern C++ library for replacing scanf and std::istream. This library attempts to move us ever so closer to replacing iostreams and C stdio altogether. It's (going to, eventually, be) faster than iostream (see Benchmarks) and type-safe, unlike scanf. Think {fmt} but in the other direction.

The library is still in early development (0.1), so don't expect perfection. See Stability for more details.

This library is not an implementation of the ISO C++ standards proposal P1729 "Text Parsing", but it's developed completely independently.

Documentation and tutorial

The documentation can be found online, from The documentation includes a tutorial.

To build the docs yourself, build the doc target generated by CMake. Building the documentation requires Doxygen.

Rather lacking examples can be found from the examples/ folder.

Should you get stuck somewhere, you can find me on the CppLang Slack, or by DMing me on Twitter.



The easiest way of integrating scnlib to your project would likely be to use a git submodule.

# Replace `external/scnlib` with the directory where you'd like the submodule be located
$ git submodule add external/scnlib

Then just do the following in your CMakeLists.txt

# Include the submodule directory
add_subdirectory(external/scnlib EXCLUDE_FROM_ALL)

# Target which you'd like to use scnlib
add_executable(my_program ...)
# Use scn::scn-header-only if you'd prefer
target_link_libraries(my_program scn::scn)

External project

You can also build scnlib separately.

# Whereever you cloned scnlib to
$ mkdir build
$ cd build
$ cmake ..
$ make -j
$ make install

Then, in your project:

# Find scnlib package
find_package(scn CONFIG REQUIRED)

# Target which you'd like to use scnlib
add_executable(my_program ...)
target_link_libraries(my_program scn::scn)

Compiler support

Every commit is tested with

  • gcc 5.5 and newer
  • clang 3.6 and newer
  • Visual Studio 2015 and 2017 with very extreme warning flags (see CMakeLists.txt) and with multiple build configurations for each compiler.

Older compilers may work, but it is not guaranteed. GCC 4.x support will not be provided, as its C++11 support is too buggy.


The master-branch will be API-stable until the next minor release, 0.2. It will receive API-compatible bugfixes, should any be necessary, tagged appropriately (0.1.1 etc).

The dev-branch will not be stable. It may be rebased, and will contain changes that may break your code. Please don't depend on it, unless you really want to live on the edge.

No ABI stability is guaranteed at this point, and none will be pre-1.0.

This is still a pre-1.0 library, so there's bound to be bugs. If you find one, please report it.


Run-time performance

These benchmarks were run on a Ubuntu 19.04 machine running kernel version 5.0.0-15, with an Intel Core i7-8750H processor, and compiled with gcc version 8.3.0, with -O3 -march=native. The source code for the benchmarks can be seen in the benchmark directory.

Times are in nanoseconds of CPU time. Lower is better.

Reading random integers

Integer type scn::scan scn::get_value std::stringstream
int 71 50 70
long long 106 81 95
unsigned 81 60 53

Reading random floating-point numbers

Floating-point type scn::scan scn::get_value std::stringstream
float 136 130 203
double 141 135 208
long double 155 143 219

Reading random whitespace-separated std::basic_strings

Character type scn::scan scn::get_value std::stringstream
char 51 48 46
wchar_t 71 80 110

Reading random characters

Character type scn::scan scn::get_value scn::getchar std::stringstream
char 20 12 5 8
wchar_t 19 12 5 15

You can run the benchmarks yourself by enabling SCN_BUILD_BENCHMARKS and building the target bench. SCN_BUILD_BENCHMARKS is enabled by default if scn is the root CMake project, and disabled otherwise.

Code size

Code size benchmarks test code bloat for nontrivial projects. It generates 25 translation units and reads values from stdin* five times to simulate a medium sized project. The resulting executable size is shown in the following tables.

The code was compiled on Kubuntu 18.10 with g++ 8.2.0. scnlib is linked dynamically to level out the playing field compared to already dynamically linked libc and libstdc++. See the directory benchmark/bloat for more information, e.g. templates for each TU.

To run these tests yourself:

$ cd build
$ make -j
$ ./benchmark/bloat/ ./benchmark/bloat

*: scn::ranges::scan scans from a Range, not from stdin

(erased) marks the usage of scn::make_erased_stream() instead of scn::make_stream().

Minimized build (-Os -DNDEBUG)

Method Executable size (KiB) Stripped size (KiB)
empty 18 14
scanf 23 18
std::istream / std::cin 25 18
scn::input 47 38
scn::input (erased) 34 26
scn::ranges::scan 94 66
scn::input (header only) 191 130
scn::input (header only & erased) 189 130
scn::ranges::scan (header only) 194 134

Release build (-O3 -DNDEBUG)

Method Executable size (KiB) Stripped size (KiB)
empty 18 14
scanf 24 18
std::istream / std::cin 30 22
scn::input 52 42
scn::input (erased) 42 34
scn::ranges::scan 80 70
scn::input (header only) 209 170
scn::input (header only & erased) 277 238
scn::ranges::scan (header only) 201 166

Debug build (-g)

Method Executable size (KiB) Stripped size (KiB)
empty 29 14
scanf 600 18
std::istream / std::cin 662 22
scn::input 1910 54
scn::input (erased) 1760 46
scn::ranges::scan 5350 554
scn::input (header only) 8460 392
scn::input (header only & erased) 8140 330
scn::ranges::scan (header only) 9300 718


The contents of this library are heavily influenced by fmtlib and its derivative works.

fmtlib is licensed under the BSD 2-clause license.
Copyright (c) 2012-2019 Victor Zverovich


Copyright (c) Elias Kosunen 2017-2019
Apache License 2.0
See LICENSE for further details

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