This was done with this script:

    #!/bin/bash

    set -eou pipefail

    # Use C++11 sstream constructor from string
    # Exclude 'ThirdParty' because it uses ossimFilename type
    find Modules/ Examples/ -not -iwholename "*ThirdParty*" -type f -print0 | xargs -0 sed -i -E "s/std::ifstream (.*)\((.*)\.c_str\(\)/std::ifstream \1\(\2/"
    find Modules/ Examples/ -not -iwholename "*ThirdParty*" -type f -print0 | xargs -0 sed -i -E "s/std::ofstream (.*)\((.*)\.c_str\(\)/std::ofstream \1\(\2/"

    # Use C++11 ::open(const std::string&) of ifstream, ofstream, etc.
    # Exclude ThirdParty because of ossimFilename type
    find Modules/ Examples/ -not -iwholename "*ThirdParty*" -type f -print0 | xargs -0 sed -i -E "s/\.open\((.*).c_str\(\)/\.open\(\1/"

    # No need for c_str when outputing to std::cout and such
    find Modules/ Examples/ -type f -print0 | xargs -0 sed -i -E "s/\.c_str\(\) ?<</ <</"

    # Use itk::ExceptionObject::SetDescription and SetLocation string versions
    find Modules/ Examples/ -type f -print0 | xargs -0 sed -i -E "s/e\.SetDescription\((.*)\.c_str\(\)\)/e\.SetDescription\(\1\)/"
    find Modules/ Examples/ -type f -print0 | xargs -0 sed -i -E "s/e\.SetLocation\((.*)\.c_str\(\)\)/e\.SetLocation\(\1\)/"

    # Use itksys::SystemTool:: string versions
    find Modules/ Examples/ -type f -print0 | xargs -0 sed -i -E "s/itksys::SystemTools::(.*)\((.*)\.c_str\(\)/itksys::SystemTools::\1\(\2/"
    # Exclude otbLogger because it uses GetCurrentDateTime which does not have a string version
    git checkout --  Modules/Core/Common/src/otbLogger.cxx
    # Exclude test/otbOGRLayerStreamStitchingFilter.cxx because there's a double use which is not compatible with the regex
    git checkout -- Modules/Segmentation/OGRProcessing/test/otbOGRLayerStreamStitchingFilter.cxx

Before this commit, many files are using std::string without including
<string>. It can work accidentally but causes issues when refactoring,
especially if using operator <<() which is included implicitly by some
compilers.

To find guilty header files, I used:

    grep -l "^ *std::string" $(grep -L "#include <string>" $(find . -type f -name "*.h"))

which finds all files containing "std::string" at the beginning of a line
(usually a member or variable declaration), but not "#include <string>".
And then this script to add the includes (plus some manual ediing):

    #!/usr/bin/env python3

    import re
    import argparse

    def fix_file(filename, header):

        with open(filename, "r") as f:
            content = f.read()

        matches = list(re.finditer(r"(#include .*\n)\n", content))
        if len(matches) == 0:
            print("no include!")
            sys.exit(-1)

        pos = matches[-1].end(1)
        open(filename, "w").write(content[:pos] + "#include <{}>\n".format(header) + content[pos:])

    if __name__ == "__main__":
        parser = argparse.ArgumentParser()
        parser.add_argument('--header', type=str, required=True)
        parser.add_argument('files', type=str, nargs='+')
        args = parser.parse_args()

        for filename in args.files:
            fix_file(filename, args.header)

STY: Change ITK_OVERRIDE for override in whole code (find Modules -type f -exec sed -i 's/ITK_OVERRIDE/override/g' {} +)