Add paddle/string/printf and tests

paddle/string/CMakeLists.txt

 cc_library(stringpiece SRCS piece.cc)
 cc_test(stringpiece_test SRCS piece_test.cc DEPS stringpiece glog gflags)
+
+cc_test(stringprintf_test SRCS printf_test.cc DEPS glog gflags)

paddle/string/printf.h

+/*
+  Copyright (c) 2016 PaddlePaddle Authors. All Rights Reserve.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+*/
+
+// Compared with std::stringstream, there are primary purpose of
+// string::Printf:
+//
+// 1. Type-safe printing, with why and how explained in
+//    http://www.drdobbs.com/stringprintf-a-typesafe-printf-family-fo/184401999.
+//    Implementation includes
+//
+//    https://github.com/c42f/tinyformat
+//    boost::format
+//    std::stringstream
+//
+//    std::stringstream is not convenient enough in many cases.  For example:
+//
+//      std::cout << std::setprecision(2) << std::fixed << 1.23456 << "\n";
+//
+//    boost::format is the most convenient one.  We can have
+//
+//      std::cout << format("%2% %1%") % 36 % 77;
+//
+//    or
+//
+//      format fmter("%2% %1%");
+//      fmter % 36; fmter % 77;
+//      std::cout << fmter.c_str();
+//
+//    But the overloading of % might be overkilling and it would be
+//    more efficient if it can write to std::cout directly.
+//
+//    tinyformat has an interface compatible with the C-printf style,
+//    and it can writes to a stream or returns a std::string:
+//
+//      std::cout << tfm::printf(
+//                  "%s, %s %d, %.2d:%.2d\n",
+//                  weekday, month, day, hour, min);
+//
+//    or
+//
+//      tfm::format(std::cout,
+//                  "%s, %s %d, %.2d:%.2d\n",
+//                  weekday, month, day, hour, min);
+//
+// 2. High-performance -- most printed strings are not too long and
+//    doens't need dynamic memory allocation.  Many StringPrintf
+//    implementations doesn't enforce type-safe, but are
+//    high-performance, including
+//
+//    https://developers.google.com/optimization/reference/base/stringprintf/
+//    https://github.com/adobe/chromium/blob/master/base/stringprintf.h
+//    https://github.com/google/protobuf/blob/master/src/google/protobuf/stubs/stringprintf.h
+//
+// According to
+// https://github.com/c42f/tinyformat#compile-time-and-code-bloat,
+// boost::format runs too slow and results in large executable binary
+// files.  So here we port tinyformat.
+
+#pragma once
+
+#include <iostream>
+#include <sstream>
+#include "paddle/string/tinyformat/tinyformat.h"  // https://github.com/c42f/tinyformat
+
+namespace paddle {
+namespace string {
+
+template <typename... Args>
+void Fprintf(std::ostream& out, const char* fmt, const Args&... args) {
+  tinyformat::vformat(out, fmt, makeFormatList(args...));
+}
+
+template <typename... Args>
+std::string Sprintf(const char* fmt, const Args&... args) {
+  std::ostringstream oss;
+  tinyformat::format(oss, fmt, args...);
+  return oss.str();
+}
+
+template <typename... Args>
+void printf(const char* fmt, const Args&... args) {
+  tinyformat::format(std::cout, fmt, args...);
+}
+
+template <typename... Args>
+void printfln(const char* fmt, const Args&... args) {
+  tinyformat::format(std::cout, fmt, args...);
+  std::cout << '\n';
+}
+
+}  // namespace string
+}  // namespace paddle

paddle/string/printf_test.cc

+#include "paddle/string/printf.h"
+
+#include <string>
+
+#include "gtest/gtest.h"
+
+TEST(StringPrintf, StringPrintf) {
+  std::string weekday = "Wednesday";
+  const char* month = "July";
+  size_t day = 27;
+  long hour = 14;
+  int min = 44;
+  EXPECT_EQ(std::string("Wednesday, July 27, 14:44"),
+            paddle::string::Sprintf(
+                "%s, %s %d, %.2d:%.2d", weekday, month, day, hour, min));
+}