Experiment with scan API

test/CMakeLists.txt

@@ -105,6 +105,7 @@ add_fmt_test(prepare-test)
 add_fmt_test(printf-test)
 add_fmt_test(custom-formatter-test)
 add_fmt_test(ranges-test)
+add_fmt_test(scan-test)
 
 # MSVC fails to compile GMock when C++17 is enabled.
 if (FMT_HAS_VARIANT AND NOT MSVC)

test/scan-test.cc

@@ -0,0 +1,87 @@
+// Formatting library for C++ - scanning API proof of concept
+//
+// Copyright (c) 2012 - present, Victor Zverovich
+// All rights reserved.
+//
+// For the license information refer to format.h.
+
+#include <array>
+#include <limits>
+
+#include "fmt/format.h"
+#include "gmock.h"
+
+namespace fmt {
+
+struct scan_arg {
+  int& value;
+  // TODO: more types
+};
+
+struct scan_args {
+  int size;
+  const scan_arg* data;
+
+  template <size_t N>
+  scan_args(const std::array<scan_arg, N>& store)
+      : size(N), data(store.data()) {
+    static_assert(N < std::numeric_limits<int>::max(), "too many arguments");
+  }
+};
+
+template <typename Handler>
+void parse_scan_format(string_view format_str, Handler& handler) {
+  const char* p = format_str.data();
+  const char* end = p + format_str.size();
+  while (p != end) {
+    char c = *p++;
+    if (c != '{' || p == end || *p++ != '}')
+      throw format_error("invalid format string");
+    handler.on_arg();
+  }
+}
+
+void vscan(string_view input, string_view format_str, scan_args args) {
+  struct handler {
+    const char* begin;
+    const char* end;
+    scan_args args;
+    int next_arg_id;
+
+    handler(string_view input, scan_args args)
+        : begin(input.data()),
+          end(begin + input.size()),
+          args(args),
+          next_arg_id(0) {}
+
+    void on_arg() {
+      int value = 0;
+      while (begin != end) {
+        char c = *begin++;
+        if (c < '0' || c > '9') throw format_error("invalid input");
+        value = value * 10 + (c - '0');
+      }
+      if (next_arg_id >= args.size)
+        throw format_error("argument index out of range");
+      args.data[0].value = value;
+    }
+  } h(input, args);
+  parse_scan_format(format_str, h);
+}
+
+template <typename... Args>
+std::array<scan_arg, sizeof...(Args)> make_scan_args(Args&... args) {
+  return {args...};
+}
+
+template <typename... Args>
+void scan(string_view input, string_view format_str, Args&... args) {
+  vscan(input, format_str, make_scan_args(args...));
+}
+}  // namespace fmt
+
+TEST(ScanTest, ReadInt) {
+  int n = 0;
+  fmt::scan("42", "{}", n);
+  EXPECT_EQ(n, 42);
+}