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Getting libstdc++ with C++17/filesystem headers on Ubuntu 18.04

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In this article, we will explore how to get libstdc++ with C++17/filesystem headers on Ubuntu 18.04. This is a common requirement for developers who are working with modern C++ applications. The process involves either updating to a newer C++ standard library or modifying the code to support older C++ standard libraries.

Quick Answer

To get libstdc++ with C++17/filesystem headers on Ubuntu 18.04, you have two options. The first option is to upgrade to a newer C++ standard library, such as g++-9. The second option is to modify your code to support older C++ standard libraries by using conditional compilation and choosing between the std::filesystem and std::experimental::filesystem namespaces.

Upgrading to a Newer C++ Standard Library

The easiest solution to get libstdc++ with C++17/filesystem headers on Ubuntu 18.04 is to upgrade to a newer C++ standard library. The newer version of Ubuntu, 20.04 (Focal Fossa), already includes the necessary headers. However, if you are required to use Ubuntu 18.04 for compatibility reasons, you can still install a newer C++ standard library on your system.

Step 1: Add the Ubuntu Toolchain PPA Repository

The first step is to add the Ubuntu Toolchain PPA repository to your system. This repository contains the newer versions of GCC and other related packages. You can add this repository by running the following commands:

sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt update

The add-apt-repository command adds the specified PPA to your system’s software sources, and the apt update command updates your system’s package index with the packages available in the newly added PPA.

Step 2: Install g++-9

Next, install g++-9 which includes a newer version of libstdc++. You can install g++-9 by running the following command:

sudo apt install g++-9

The apt install command installs the specified package along with its dependencies.

After the installation, you can compile your code using g++-9 instead of the default g++ compiler. This will ensure that your code is compiled with the newer version of libstdc++.

Modifying the Code to Support Older C++ Standard Libraries

If you prefer not to upgrade to a newer C++ standard library, you can modify your code to support older C++ standard libraries. This process involves changing the code to choose between the std::filesystem and std::experimental::filesystem namespaces depending on the availability of the header.

Step 1: Modify the Code

Here’s an example of how you can modify your code:

#if __has_include(<filesystem>)
 #include <filesystem>
 namespace fs = std::filesystem;
#elif __has_include(<experimental/filesystem>)
 #include <experimental/filesystem> 
 namespace fs = std::experimental::filesystem;
#else
 #error "Missing the <filesystem> header."
#endif

In this code, the __has_include directive checks if the specified header is available. If the <filesystem> header is available, it is included and the fs namespace is set to std::filesystem. If the <filesystem> header is not available but the <experimental/filesystem> header is, the latter is included and the fs namespace is set to std::experimental::filesystem. If neither header is available, an error message is displayed.

Step 2: Use the Appropriate Compilation Flags

After modifying the code, you need to use the appropriate compilation flags depending on the C++ standard library you are using. For GNU libstdc++ prior to 9.1, use the flag -lstdc++fs as the last flag. For LLVM libc++ prior to 9.0, use the flag -lc++fs as the last flag.

Here are some example compilation commands:

g++ -std=c++17 your_code.cpp -lstdc++fs
clang++ -std=c++17 your_code.cpp -lstdc++fs
clang++ -std=c++17 -stdlib=libc++ -lc++abi your_code.cpp -lstdc++fs

In these commands, the -std=c++17 flag sets the C++ standard to C++17, and the -lstdc++fs or -lc++fs flag links the filesystem library.

If you are using CMake, add the following line after add_executable():

target_link_libraries(project_name_here stdc++fs) // for GNU libstdc++
target_link_libraries(project_name_here c++fs) // for LLVM libc++

In these lines, the target_link_libraries command links the specified library to the target executable.

Conclusion

Getting libstdc++ with C++17/filesystem headers on Ubuntu 18.04 involves either upgrading to a newer C++ standard library or modifying your code to support older C++ standard libraries. Both methods have their pros and cons, so choose the one that best fits your requirements. Remember to review the documentation for any potential changes in behavior when switching between std::filesystem and std::experimental::filesystem.

What is `libstdc++`?

libstdc++ is the standard C++ library implementation provided by the GNU Compiler Collection (GCC). It includes various components such as containers, algorithms, input/output operations, and support for C++ language features.

Why do I need C++17/filesystem headers?

C++17 introduced the <filesystem> header, which provides a standardized way to manipulate files and directories in C++. This header simplifies file system operations and provides a more modern and robust interface compared to older approaches.

Can I use C++17/filesystem headers on Ubuntu 18.04 without upgrading the C++ standard library?

Yes, you can modify your code to support older C++ standard libraries by using conditional compilation and choosing between the std::filesystem and std::experimental::filesystem namespaces depending on the availability of the header.

What is the difference between `std::filesystem` and `std::experimental::filesystem`?

std::filesystem is the standardized version of the file system library introduced in C++17. It is preferred for new code and provides a stable API. std::experimental::filesystem is the experimental version of the library that was available before C++17. It may have some differences and limitations compared to the standardized version.

How can I upgrade to a newer C++ standard library on Ubuntu 18.04?

You can upgrade to a newer C++ standard library on Ubuntu 18.04 by adding the Ubuntu Toolchain PPA repository to your system and installing a newer version of g++, such as g++-9, which includes a newer version of libstdc++.

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