C++ Standard

¹

Data source: www.jetbrains.com

CMake²

• Cross-platform, compiler-independent
• Generates configuration for other build systems
• Open-Source

Example

File: main.cpp

#include <add.h>

int main() {
  return add(2, 3);
}

Header File

File: add.h

#pragma once

int add(const int x, const int y);

Translation Unit

File: add.cpp

#include <add.h>

int add(const int x, const int y) {
  return x + y;
}

Interface

• Describes parameters and return value
• Is checked at callsite (main.cpp) by the compiler
• Promises an implementation

Implementation

• Specifies what function does
• Not checked at callsite (main.cpp) by the compiler
• Only one implementation allowed

Quiz

How does the build system know which header belongs to which translation unit?

Build Process

• Compiler
• Linker

Compile Step

cd src && mkdir build && cd build
g++ -c ../add.cpp -o add.o
g++ -c ../main.cpp -o main.o

Compiler’s View

File: main.cpp

int add(const int x, const int y);

int main() {
  return add(2, 3);
}

Compile Step Characteristics

• Optimizes code, slow
• Checks external interfaces
• Does not check external implementations
• Parallelizable (multiple translation units at once)

Link Step

g++ add.o main.o -o example

Linker’s View

• File: main.o
  • Provides int main()
  • Needs int add(int, int)
• File add.o
  • Provides int add(int, int)

Link Step Characteristics

• Does not optimize code, fast (traditionally)
• Matches needed with provided symbols

Run the executable

./example
echo $?

Rebuild can become cumbersome

Change main.cpp:

#include <add.h>
#include <iostream>

int main() {
  std::cout << add(1, 2) << std::endl;
  return 0;
}

Rebuild:

# add.o: not changed / already built
g++ -c ../main.cpp -o main.o
g++ add.o main.o -o example

Special Case

Sometimes callsite needs definition.

• Inline function

inline int add(const int x, const int y) { return x + y; }

• Template

template <class T>
T add(const T x, const T y) { return x + y; }

Libraries

• Provide interfaces for compiler
• Provide implementation for linker (.a, .so, …)

Summary

• Building is system dependent
• External libraries may require flags
• Managing flags and parameters becomes infeasible

Goals

• Targets, targets, targets
• Declarative
• Avoid global variables

Example

cmake_minimum_required(VERSION 3.16 FATAL_ERROR)
project(addition VERSION 0.1.0 LANGUAGES C CXX)

add_library(addition addition/add.cpp)
target_include_directories(addition PUBLIC addition/include)

add_executable(example main.cpp)
target_link_libraries(example PRIVATE addition)

Targets

• addition (library)
• example (executable)

add_library(addition addition/add.cpp)
target_include_directories(addition PUBLIC addition/include)

add_executable(example main.cpp)
target_link_libraries(example PRIVATE addition)

Quiz

What if addition depends on a library XYZ?

Solution

• Link XYZ to addition.

target_link_libraries(addition PUBLIC XYZ)

• Target example automatically depends on XYZ.

Visibility

Library Types

BUILD_SHARED_LIBS defines default.

add_library(addition STATIC addition/add.cpp)
add_library(addition SHARED addition/add.cpp)

Localize Settings

Library knows paths and flags.

target_link_libraries(example PRIVATE addition)

Using External Library

Library knows paths and flags.

find_package(Eigen3 3.3.7 REQUIRED)

target_link_libraries(example PRIVATE Eigen3::Eigen)

Find Package Modes

• CONFIG mode: library maintainer provides settings
• MODULE mode: a provided program tries to detect settings
• MANUAL mode: 🙁

Find Package Modes

• Eigen3: CONFIG mode 😎

find_package(Eigen3 3.3.7 CONFIG REQUIRED)

• HDF5: MODULE mode 🙂

find_package(HDF5 1.10 MODULE REQUIRED)

Find Package Modes

• PETSc: MANUAL mode 🙁

  • Write find module
  • PkgConfig may help

Global Variables

Use targets, not global variables.

• set(CMAKE_CXX_FLAGS "-std=c++11") 😭
• target_compile_features(addition PUBLIC cxx_std_11) 😎

Calling CMake

Platform-independent builds.

Configure:

cmake -DCMAKE_BUILD_TYPE=Debug -S src -B build -G Ninja
# -DCMAKE_BUILD_TYPE=...  Build type
# -S                      Source directory
# -B                      Build directory
# -G                      Build system generator

Build:

cmake --build build
# --build                 Build directory

Build types

More info: Manual

Beyond Building

• Testing
• Package libraries
• Generate installers