How to make cmake output to the "build" directory? - c++

Question

I'm using cmake to compile a C++ project, and I want cmake to generate all the output files(metafiles like Makefile used to create binaries) in the build folder. I've checked all the answers in How do I make cmake output into a 'bin' dir?, none of them worked for me(suprisingly!). Files are generated in the root folder instead of in the build folder, what's wrong here? I guess I must have missed something.
Code Structure
➜ cmake-test tree .
.
├── CMakeLists.txt
└── hello.cpp
0 directories, 2 files
CMakeLists.txt
# Specify the minimum version for CMake
cmake_minimum_required(VERSION 3.11)
# Project's name
project(hello)
# Set the output folder where your program will be created
set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/build)
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/build)
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
# The following folder will be included
include_directories("${PROJECT_SOURCE_DIR}")
add_executable(hello ${PROJECT_SOURCE_DIR}/hello.cpp)
Build Commands and Outputs
➜ cmake-test cmake .
-- The C compiler identification is GNU 8.2.0
-- The CXX compiler identification is GNU 8.2.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /home/searene/CLionProjects/cmake-test
➜ cmake-test ls
bin CMakeCache.txt CMakeFiles cmake_install.cmake CMakeLists.txt hello.cpp Makefile
cmake version
➜ cmake-test cmake --version
cmake version 3.11.4
CMake suite maintained and supported by Kitware (kitware.com/cmake).
OS
Linux

Answer 1

The usual way to do this, rather than changing variables to set the path, is simply to create the output directory, change to it, and run cmake from there. So instead of cmake . you usually have cmake .. or similar.
I understand the initial impulse to say "But I expect my build system to write output somewhere else." But CMake is not usually used in the way you were initially expecting, and other people who run your CMake build won't expect what you were expecting, so it's probably best to just use the built-in, default behavior, which is to put the output wherever cmake was run.
Put another way: You are fighting against the tool. Don't do that.

Answer 2

Disclaimer: I recommend going with #john-zwinck's answer.
By default, cmake uses the current working directory as build directory and whatever path you provide as source directory. So the normal way of achieving your goal is
create the build directory (mkdir build)
go there (cd build)
call cmake with the source dir as argument (cmake path/to/source)
BUT there is another way, as far as I know not documented in the cmake docs and only kept for compatibility reasons or internal usage, that some people are using. The -B and -H flags
cmake -Hpath/to/source -Bpath/to/build
or even from the source dir
cmake . -Bbuild
Important: no space after -B.

Answer 3

CMake 3.19.1 (not sure how about older ones) has following option (from docs):
cmake [<options>] -S <path-to-source> -B <path-to-build>
Uses as the build tree and as the
source tree. The specified paths may be absolute or relative to the
current working directory. The source tree must contain a
CMakeLists.txt file. The build tree will be created automatically if
it does not already exist. For example:
cmake -S src -B build