No template named 'unary_function' in namespace 'std'; did you mean '__unary_function'? – Ios

The Solutions:

Solution 1: Fix Xcode error

In this case, you can simply click on “Fix” in Xcode after the build fails. This will automatically change the `std::unary_function` to `std::__unary_function`, which is the correct type name in C++11 and later.

Solution 2: Fixing the Error in Xcode

To resolve the error “No template named ‘unary_function’ in namespace ‘std’; did you mean ‘__unary_function’?” in Xcode, follow these steps:

1. Open Xcode and Launch the Simulator:
   Open Xcode and click on the play icon to launch the simulator. The simulator may fail to launch, but Xcode will display the error in the sidebar.

2. Identify the Error:
   Click on the error in the sidebar to see where the error is located in the code. This will help you understand the specific issue causing the error.

3. Click the "Fix" Button:
   Once you’ve identified the error, click the "Fix" button in Xcode. This feature in Xcode automatically attempts to resolve the error by making appropriate changes to the code.

4. Build and Run:
   After clicking "Fix," Xcode will attempt to fix the error and rebuild the project. If successful, the simulator will launch successfully, and the error will be resolved.

Solution 3: Use libcpp Function

Until the issue is fixed in the source files, the most effective solution is to add the removed libcpp function to your podfile, as outlined in this GitHub discussion: https://github.com/facebook/react-native/issues/37748#issuecomment-1580589448.

The following code snippet demonstrates how to add the libcpp function to your podfile.

post_install do |installer|
  installer.pods_project.targets.each do |target|
    target.build_configurations.each do |config|
      config.build_settings['GCC_PREPROCESSOR_DEFINITIONS'] ||= ['$(inherited)', '_LIBCPP_ENABLE_CXX17_REMOVED_UNARY_BINARY_FUNCTION']
    end
  end
end

By adding this code snippet to your podfile, you can enable the removed libcpp function and resolve the error caused by the missing ‘unary_function’ template in the ‘std’ namespace.

Solution 4: Use std::__unary_function Instead of std::unary_function

In certain versions of Xcode (__gt 15.0), the `std::unary_function` template has probably been renamed to
`std::__unary_function`. To resolve the issue, you can replace `std::unary_function` with `std::__unary_function` in your code. This will allow your code to work with the latest version of Xcode.

>struct hash_base : std::__unary_function<T, std::size_t> ;

Solution 5: Add Macro

Explanation:

To resolve the error "No template named ‘unary_function’ in namespace ‘std’; did you mean ‘__unary_function’?" when upgrading to Xcode 15.0, update library code to use std::function and std::bind instead of unary_function. As a workaround, in Xcode, under Pods > Build Settings > Apple Clang - Preprocessing > Macro:

• Add _LIBCPP_ENABLE_CXX17_REMOVED_UNARY_BINARY_FUNCTION in both Release and Debug configurations.

Code Snippet:

- Add this line in Xcode:
Pods > Build Settings > Apple Clang - Preprocessing > Macro
Release & Debug: _LIBCPP_ENABLE_CXX17_REMOVED_UNARY_BINARY_FUNCTION

Pros:

• This is a straightforward fix that can be easily implemented.
• It does not require any major code changes.

Cons:

• It may not be necessary for all projects.
• It may not be compatible with all versions of Xcode.

Video Explanation: