How to mock Non-virtual Methods in concrete classes using gmock?

Question

I somehow extended the gmock test case from donsoft.io's example, and made it as follows:

coinflipper/
├── BUILD
├── WORKSPACE
├── coinflipper.cc
├── coinflipper.h
├── rng.cc
└── rng.h

Well, instead put the Rng class as a parameter of the constructor of the CoinFlipper, I made it initialized inside the CoinFlipper::flipCoin() method.

I was wondering how to mock the generate() from Rng in this case?

coinflipper.cc

#include "coinflipper.h"
#include <iostream>

CoinFlipper::CoinFlipper() {}

CoinFlipper::Result CoinFlipper::flipCoin() const {
  Rng d_rng;
  const double val = d_rng.generate(0.0, 1.0);
  return (val < 0.5) ? HEADS : TAILS;
}

int main(int argc, char** argv) {
  CoinFlipper cf;
  CoinFlipper::Result res = cf.flipCoin();
  if (res == 0) {
      std::cout << "head" << std::endl;
  } else {
      std::cout << "tail" << std::endl;
  }
  return 0;
}

coinflipper.h

#include "rng.h"

class CoinFlipper {
 public:
  enum Result { HEADS = 0, TAILS = 1 };

  explicit CoinFlipper();
  Result flipCoin() const;
};

rng.h

#ifndef RNG_H
#define RNG_H
class Rng {
 public:
  Rng() {};
  ~Rng() {};
  double generate(double min, double max);
};
#endif

rng.cc

#include "rng.h"

double Rng::generate(double min, double max) {    
    return 0.75; //Just for test
}

BUILD

load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library")

cc_library(
    name = "rng",
    srcs = ["rng.cc"],
    hdrs = ["rng.h"],
)

cc_binary(
    name = "coinflipper",
    srcs = ["coinflipper.cc", "coinflipper.h"],    
    deps = [
        ":rng",
    ],
)

cc_test(
  name = "coinflipper_test",
  size = "small",
  srcs = ["coinflipper_test.cc", "rng.h","coinflipper.h","coinflipper.cc"],
  deps = ["@com_google_googletest//:gtest_main"],
)

coinflipper_test.cc

#include "coinflipper.h"
#include "gmock/gmock.h"
#include "gtest/gtest.h"

class MockRng {
 public:  
  MOCK_METHOD2(generate, double(double, double));
};

TEST(CoinFlipper, ShouldReturnHeadsIfRandValueIsLessThanProbability) {
  MockRng rng;
  EXPECT_CALL(rng, generate(::testing::DoubleEq(0.0), ::testing::DoubleEq(1.0)))
      .Times(::testing::Exactly(1))
      .WillOnce(::testing::Return(0.25));

  CoinFlipper coinFlipper;
  auto result = coinFlipper.flipCoin<MockRng>(rng);
  EXPECT_EQ(CoinFlipper::HEADS, result);
}

Bazel output:

$ bazel test --test_output=all //:coinflipper_test
DEBUG: Rule 'rules_cc' indicated that a canonical reproducible form can be obtained by modifying arguments sha256 = "56ac9633c13d74cb71e0546f103ce1c58810e4a76aa8325da593ca4277908d72"
DEBUG: Repository rules_cc instantiated at:
  /Users/pvd/Downloads/toys/cpp/bazelgtest/coinflipper/WORKSPACE:9:13: in <toplevel>
Repository rule http_archive defined at:
  /private/var/tmp/_bazel_pvd/3077b447e558b1418694504407cbcb45/external/bazel_tools/tools/build_defs/repo/http.bzl:336:31: in <toplevel>
DEBUG: Rule 'com_google_googletest' indicated that a canonical reproducible form can be obtained by modifying arguments sha256 = "5cf189eb6847b4f8fc603a3ffff3b0771c08eec7dd4bd961bfd45477dd13eb73"
DEBUG: Repository com_google_googletest instantiated at:
  /Users/pvd/Downloads/toys/cpp/bazelgtest/coinflipper/WORKSPACE:3:13: in <toplevel>
Repository rule http_archive defined at:
  /private/var/tmp/_bazel_pvd/3077b447e558b1418694504407cbcb45/external/bazel_tools/tools/build_defs/repo/http.bzl:336:31: in <toplevel>
INFO: Analyzed target //:coinflipper_test (0 packages loaded, 0 targets configured).
INFO: Found 1 test target...
FAIL: //:coinflipper_test (see /private/var/tmp/_bazel_pvd/3077b447e558b1418694504407cbcb45/execroot/__main__/bazel-out/darwin-fastbuild/testlogs/coinflipper_test/test.log)
INFO: From Testing //:coinflipper_test:
==================== Test output for //:coinflipper_test:
dyld: lazy symbol binding failed: Symbol not found: __ZN3Rng8generateEdd
  Referenced from: /private/var/tmp/_bazel_pvd/3077b447e558b1418694504407cbcb45/sandbox/darwin-sandbox/72/execroot/__main__/bazel-out/darwin-fastbuild/bin/coinflipper_test.runfiles/__main__/coinflipper_test
  Expected in: flat namespace

dyld: Symbol not found: __ZN3Rng8generateEdd
  Referenced from: /private/var/tmp/_bazel_pvd/3077b447e558b1418694504407cbcb45/sandbox/darwin-sandbox/72/execroot/__main__/bazel-out/darwin-fastbuild/bin/coinflipper_test.runfiles/__main__/coinflipper_test
  Expected in: flat namespace

================================================================================
Target //:coinflipper_test up-to-date:
  bazel-bin/coinflipper_test
INFO: Elapsed time: 0.429s, Critical Path: 0.14s
INFO: 2 processes: 2 darwin-sandbox.
INFO: Build completed, 1 test FAILED, 2 total actions
//:coinflipper_test                                                      FAILED in 0.1s
  /private/var/tmp/_bazel_pvd/3077b447e558b1418694504407cbcb45/execroot/__main__/bazel-out/darwin-fastbuild/testlogs/coinflipper_test/test.log

INFO: Build completed, 1 test FAILED, 2 total actions

Answer 1

Define dependencies(The random generator here) as local variables are not recommended, it's much harder to do dependencies injection(Or it won't be possible), so I change the functions Rng_t into template function and pass the Rng as a parameter.

In practice to construct a random generation may be heavy work, it needs to initialize its internal status, to construct it every time we call the function flipCoin is waste.

The non-virtual function can be mocked, one most commonly used strategy is to use the template, here we make the class CoinFlipper's member function as a template function, then we can test the dependency with our MockRng.

Be aware that for the template function, we need to define the member function in the header file.

coinflipper.h:

#pragma once
#include "rng.h"

class CoinFlipper {
 public:
  enum Result { HEADS = 0, TAILS = 1 };

  template <typename Rng_t>
  Result flipCoin(Rng_t& rng) {
    const double val = rng.generate(0.0, 1.0);
    return (val < 0.5) ? HEADS : TAILS;
  }
};

The test file part, MockRng doesn't inherit anything now. And the test member function we use here has the type CoinFlipper::flipCoin<MockRng>. For production code: we use the type CoinFlipper::flipCoin<Rng>

//#include "mockrng.h"

#include "coinflipper.h"
#include "gmock/gmock.h"
#include "gtest/gtest.h"

class MockRng {
 public:
  MOCK_METHOD2(generate, double(double, double));
};

TEST(CoinFlipper, ShouldReturnHeadsIfRandValueIsLessThanProbability) {
  MockRng rng;
  EXPECT_CALL(rng, generate(::testing::DoubleEq(0.0), ::testing::DoubleEq(1.0)))
      .Times(::testing::Exactly(1))
      .WillOnce(::testing::Return(0.25));

  CoinFlipper coinFlipper;
  auto result = coinFlipper.flipCoin<MockRng>(rng);
  EXPECT_EQ(CoinFlipper::HEADS, result);
}

See related question here:

Mock non-virtual method C++ (gmock)

The official document:

https://chromium.googlesource.com/external/github.com/google/googletest/+/refs/tags/release-1.8.0/googlemock/docs/CookBook.md