如何使用 googletest 捕获 stdout/stderr？

Question

使用 googletest 框架时是否可以捕获 stdout 和 stderr？

例如，我想调用一个将错误写入控制台（stderr）的函数。 现在，当在测试中调用该函数时，我想断言那里没有输出。

或者，也许我想测试错误行为，并想断言当我（故意）产生错误时会打印某个字符串。

Answer 1

Googletest 为此提供了函数：

testing::internal::CaptureStdout();
std::cout << "My test";
std::string output = testing::internal::GetCapturedStdout();

Answer 2

我之前曾在测试输出时使用此代码片段将 cout 调用重定向到字符串流。希望它能激发一些想法。我以前从未使用过 googletest。

// This can be an ofstream as well or any other ostream
std::stringstream buffer;

// Save cout's buffer here
std::streambuf *sbuf = std::cout.rdbuf();

// Redirect cout to our stringstream buffer or any other ostream
std::cout.rdbuf(buffer.rdbuf());

// Use cout as usual
std::cout << "Hello World";

// When done redirect cout to its old self
std::cout.rdbuf(sbuf);

在重定向回原始输出之前，请使用谷歌测试检查缓冲区中的输出。

Answer 3

避免这样做始终是一个好的设计理念。如果您确实想这样做，请执行以下操作：

#include <cstdio>
#include <cassert>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <iostream>

int main() {
   int fd = open("my_file.log", O_WRONLY|O_CREAT|O_TRUNC, 0660);
   assert(fd >= 0);
   int ret = dup2(fd, 1);
   assert(ret >= 0);
   printf("This is stdout now!\n");
   std::cout << "This is C++ iostream cout now!" << std::endl;
   close(fd);
}

要使用 stderr 而不是 stdout，请将 dup2 的第二个参数更改为 2。为了不通过文件进行捕获，您可以使用管道对。

Answer 4

不要这样做，而是使用依赖项注入来删除对 std::cout 的直接使用。在您的测试代码中，使用 std:ostringstream 类的模拟对象作为模拟对象，而不是真正的 std::cout。

所以代替这个：

 void func() {
    ...
    std::cout << "message";
    ...
 }

 int main (int argc, char **argv) {
    ...
    func();
    ...
 }

有这个：

 void func(std::ostream &out) {
    ...
    out << "message";
    ...
 }

 int main(int argc, char **argv) {
    ...
    func(std::cout);
    ...
 }

Answer 5

将 Wgaffa 的建议（我喜欢）放入 Google 测试装置中，人们可能会这样写：

namespace {

    class MyTestFixture : public ::testing::Test {
    protected:
        MyTestFixture() : sbuf{nullptr} {
            // intentionally empty
        }

        ~MyTestFixture() override = default;

        // Called before each unit test
        void SetUp() override {
            // Save cout's buffer...
            sbuf = std::cout.rdbuf();
            // Redirect cout to our stringstream buffer or any other ostream
            std::cout.rdbuf(buffer.rdbuf());
        }

        // Called after each unit test
        void TearDown() override {
            // When done redirect cout to its old self
            std::cout.rdbuf(sbuf);
            sbuf = nullptr;
        }

        // The following objects can be reused in each unit test

        // This can be an ofstream as well or any other ostream
        std::stringstream buffer{};
        // Save cout's buffer here
        std::streambuf *sbuf;
    };

    TEST_F(MyTestFixture, StackOverflowTest) {
        std::string expected{"Hello"};
        // Use cout as usual
        std::cout << expected;
        std::string actual{buffer.str()};
        EXPECT_EQ(expected, actual);
    }
} // end namespace

Answer 6

根据 Wgaffa 的答案，我制作了这个辅助类，可以使用 std::cout 或 std::cerr 构造：

class CaptureHelper
{
public:
  CaptureHelper(std::ostream& ioStream)
    : mStream(ioStream),
    mIsCapturing(false)
  { }

  ~CaptureHelper()
  {
    release();
  }

  void capture()
  {
    if (!mIsCapturing)
    {
      mOriginalBuffer = mStream.rdbuf();
      mStream.rdbuf(mRedirectStream.rdbuf());
      mIsCapturing = true;
    }
  }

  std::string release()
  {
    if (mIsCapturing)
    {
      std::string wOutput = mRedirectStream.str();
      mStream.rdbuf(mOriginalBuffer);
      mIsCapturing = false;
      return wOutput;
    }
  }

private:
  std::ostream& mStream;
  bool mIsCapturing;
  std::stringstream mRedirectStream;
  std::streambuf* mOriginalBuffer;

};

Answer 7

这是另一个实用程序类，它执行接受的答案建议的操作：

class ScopedOStreamCapture {
 public:
  explicit ScopedOStreamCapture(std::ostream& stream)
      : stream_(stream), old_(stream.rdbuf()) {
    stream_.rdbuf(buffer_.rdbuf());
  }

  ~ScopedOStreamCapture() { stream_.rdbuf(old_); }

  ScopedOStreamCapture(const ScopedOStreamCapture&) = delete;
  ScopedOStreamCapture(ScopedOStreamCapture&&) = delete;
  ScopedOStreamCapture& operator=(const ScopedOStreamCapture&) = delete;
  ScopedOStreamCapture& operator=(ScopedOStreamCapture&&) = delete;

  std::string Consume() {
    std::stringstream tmp;
    tmp.rdbuf()->swap(*buffer_.rdbuf());
    return tmp.str();
  }

  std::string Peek() {
    return buffer_.str();
  }

 private:
  std::ostream& stream_;
  std::streambuf* old_;
  std::stringstream buffer_;
};

用法：

TEST(ScopedOStreamCaptureTest, CapturesErr) {
  std::cerr << "Not captured before!" << std::endl;
  {
    ScopedOStreamCapture capture(std::cerr);
    std::cerr << "hello";
    EXPECT_EQ(capture.Peek(), "hello");
    std::cerr << " world";
    EXPECT_EQ(capture.Peek(), "hello world");
    EXPECT_EQ(capture.Consume(), "hello world");
    EXPECT_EQ(capture.Peek(), "");
  }
  std::cerr << "Not captured after!" << std::endl;
}

在 Godbolt 中运行

Answer 8

我们正在做的正是你所指的。

首先我们创建了一些宏：

    #define CAPTURE_STDOUT StdoutRedirect::instance().redirect();
    #define RELEASE_STDOUT StdoutRedirect::instance().reset();
    #define ASSERT_INFO( COUNT, TARGET )   \
      ASSERT_PRED_FORMAT2(OurTestPredicates::AssertInfoMsgOutput, TARGET, COUNT );

请参阅此答案以捕获 stdout 和 stderr：
https://stackoverflow.com/a/5419409/9796918
只需使用它们的BeginCapture()、EndCapture() 代替我们的redirect() 和reset()。

在 AssertInfoMsgOutput 方法中：

    AssertionResult OurTestPredicates::AssertInfoMsgOutput( const char* TARGET,
        const char* d1,
        const char* d2,
        int         COUNT )
    {
      int count = 0;
      bool match = false;
      std::string StdOutMessagge = GetCapture();
      // Here is where you process the stdout/stderr info for the TARGET, and for
      // COUNT instances of that TARGET message, and set count and match
      // appropriately
      ...
      if (( count == COUNT ) && match )
      {
        return ::testing::AssertionSuccess();
      }
      return :: testing::AssertionFailure() << "not found";
    }

现在，在单元测试中，只需将要捕获 stdout/stderr 的调用包装起来：

    CAPTURE_STDOUT
    // Make your call to your code to test / capture here
    ASSERT_INFO( 1, "Foo bar" );
    RELEASE_STDOUT