enable freopen to make some guides on the internet work

those two multiply the output into a file:

void redirect_output(const char *filename);
void redirect_output(std::string const &filename)

those two read everything from a file:

redirect_input(const char *filename);
redirect_input(std::string const& filename);

both can be resetted using:

void reset_input();
reset_output();

example.cpp

@@ -27,6 +27,9 @@ signed main() {
   a.foo();
   a.bar(3,4,"hi");
 
+  print("this should not be in the output file");
+  redirect_output("example.out");
+
   auto [i, j] = read<int, int>();
   print(i, j);
 
@@ -68,4 +71,12 @@ signed main() {
   print(map<int, string>{{3,"three"},{1,"one"}});
   print(tuple{unordered_map<int, string>{{3,"three"},{1,"one"}}});
   print(unordered_set<int>{3,1,4});
+
+  redirect_input("example.in");
+  print("reading from example.in:", read_string());
+  redirect_input("example.in");
+  print("reading from example.in:", read_string());
+  print("this is the last line on example.out");
+  reset_output();
+  print("this is stdout only");
 }

include/bits/soi-deprecate.hpp

@@ -31,7 +31,6 @@ snprintf(char *buffer, std::size_t buf_size, const char *format, ...);
 #define puts puts_is_evil
 #define fputs fputs_is_evil
 #define fgets fgets_is_evil
-#define freopen freopen_is_evil
 #define fopen fopen_is_evil
 #define fclose fclose_is_evil
 #define fflush fflush_is_evil
@@ -48,8 +47,6 @@ snprintf(char *buffer, std::size_t buf_size, const char *format, ...);
 [[deprecated("<cstdio> is evil.  Use cin/cout.")]] char *
 fgets(char *str, int count, FILE *stream);
 [[deprecated("<cstdio> is evil.  Use cin/cout.")]] std::FILE *
-freopen(const char *filename, const char *mode, std::FILE *stream);
-[[deprecated("<cstdio> is evil.  Use cin/cout.")]] std::FILE *
 fopen(const char *filename, const char *mode);
 [[deprecated("<cstdio> is evil.  Use cin/cout.")]] int
 fclose(std::FILE *stream);

include/bits/soi-redirect.hpp

@@ -0,0 +1,93 @@
+#include <ostream>
+#include <streambuf>
+
+namespace soi {
+
+namespace redirect {
+
+class teebuf : public std::streambuf {
+  std::streambuf *sb1_;
+  std::streambuf *sb2_;
+
+  int overflow(int c) {
+    using traits = std::streambuf::traits_type;
+    bool rc = true;
+    if (!traits::eq_int_type(traits::eof(), c)) {
+      traits::eq_int_type(this->sb1_->sputc(c), traits::eof()) && (rc = false);
+      traits::eq_int_type(this->sb2_->sputc(c), traits::eof()) && (rc = false);
+    }
+    return rc ? traits::not_eof(c) : traits::eof();
+  }
+  int sync() {
+    bool rc = false;
+    this->sb1_->pubsync() != -1 || (rc = false);
+    this->sb2_->pubsync() != -1 || (rc = false);
+    return rc ? -1 : 0;
+  }
+
+public:
+  teebuf(std::streambuf *sb1, std::streambuf *sb2) : sb1_(sb1), sb2_(sb2) {}
+};
+
+static std::streambuf *checked_rdbuf(std::ios &stream, const char *filename,
+                                     const char *action) {
+  if (!stream) {
+    std::cerr << "ERROR: can't open to file " << filename << " for " << action
+              << '\n';
+    std::exit(-1);
+  }
+  return stream.rdbuf();
+}
+
+struct output_redirector {
+  std::ofstream fout;
+  teebuf tee;
+  std::streambuf *old_cout;
+
+  output_redirector(const char *filename)
+      : fout(filename),
+        tee(checked_rdbuf(fout, filename, "writing"), std::cout.rdbuf()),
+        old_cout(std::cout.rdbuf(&tee)) {}
+
+  ~output_redirector() { std::cout.rdbuf(old_cout); }
+};
+
+struct input_redirector {
+  std::ifstream fin;
+  std::streambuf *old_cin;
+
+  input_redirector(const char *filename)
+      : fin(filename),
+        old_cin(std::cin.rdbuf(checked_rdbuf(fin, filename, "reading"))) {}
+
+  ~input_redirector() { std::cin.rdbuf(old_cin); }
+};
+
+std::forward_list<output_redirector> output_redirectors;
+std::unique_ptr<input_redirector> the_input_redirector;
+
+} // namespace redirect
+
+} // namespace soi
+
+void redirect_output(const char *filename) {
+  soi::redirect::output_redirectors.emplace_front(filename);
+}
+void redirect_output(std::string const &filename) {
+  redirect_output(filename.c_str());
+}
+void reset_output() {
+  soi::redirect::output_redirectors.clear();
+}
+
+void reset_input() {
+  soi::redirect::the_input_redirector.reset();
+}
+void redirect_input(const char *filename) {
+  reset_input();
+  auto next = new soi::redirect::input_redirector(filename);
+  soi::redirect::the_input_redirector.reset(next);
+}
+void redirect_input(std::string const &filename) {
+  redirect_input(filename.c_str());
+}

include/soi

@@ -77,7 +77,7 @@ soi_initializer soi_initializer_{false};
 } // end namespace soi
 
 #include "bits/soi-io.hpp"
-
+#include "bits/soi-redirect.hpp"
 #include "bits/soi-deprecate.hpp"
 
 #define int int64_t