使用Bazel运行Google Test遇`symbol not found in flat namespace`错误

项目背景 #

我之前是Python开发者，最近开始学习C++和Bazel构建系统，搭建了一个用于熟悉Google Test的简单测试项目，执行测试时遇到符号未找到错误，具体信息及项目代码如下：

项目文件结构 #

.
  WORKSPACE
  main/
    shape.cc
    shape.h
    BUILD
  test/
    shape_test.cc
    BUILD

各文件内容 #

WORKSPACE文件

# WORKSPACE file - from http://google.github.io/googletest/quickstart-bazel.html

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

http_archive(
  name = "com_google_googletest",
  urls = ["https://github.com/google/googletest/archive/609281088cfefc76f9d0ce82e1ff6c30cc3591e5.zip"],
  strip_prefix = "googletest-609281088cfefc76f9d0ce82e1ff6c30cc3591e5",
)

main/BUILD文件

# main/BUILD

load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library")
package(default_visibility = ["//visibility:public"])

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

main/shape.h文件

// main/shape.h

#include <string>
#include <vector>
class AbstractShape
{
protected:
    float area;
    float perimeter;
    std::string shape_name;

public:
    float get_area();
    void describe();
    virtual std::string get_description();
};

AbstractShape factory_method(float h, float w, std::string colour = "", std::string surface = "");

float calculate_area(std::vector<AbstractShape> shape_arr);

class Rectangle : public AbstractShape
{

public:
    Rectangle(float h, float w);
    std::string get_description();
};

class ColourAndSurfaceRectangle : public AbstractShape
{
private:
    std::string colour;
    std::string surface;

public:
    ColourAndSurfaceRectangle(float h, float w, std::string colour = "", std::string shape = "");
    std::string get_description();
};

main/shape.cc文件

// main/shape.cc

#include "shape.h"
#include <vector>
#include <string>
#include <iostream>

float calculate_area(std::vector<AbstractShape> shape_arr)
{
    float sum = 0;
    for (AbstractShape shape : shape_arr)
    {
        sum += shape.get_area();
    }
    return sum;
}

float AbstractShape::get_area()
{
    return area;
}

void AbstractShape::describe()
{
    std::string description = this->get_description();
    std::cout << description << std::endl;
}

Rectangle::Rectangle(float h, float w)
{
    this->shape_name = "Rectangle";
    this->area = h * w;
    this->perimeter = 2 * (h + w);
}

std::string Rectangle::get_description()
{
    std::string description = "Shape:" + this->shape_name + "\n" +
                              "Perimeter: " + std::to_string(this->perimeter) + "\n" +
                              "Area: " + std::to_string(this->area) + "\n";

    return description;
}

ColourAndSurfaceRectangle::ColourAndSurfaceRectangle(float h, float w, std::string colour, std::string shape)
{
    this->shape_name = "Rectangle";
    this->area = h * w;
    this->perimeter = 2 * (h + w);
    this->colour = colour;
    this->surface = surface;
}

std::string ColourAndSurfaceRectangle::get_description()
{
    std::string description = "Shape:" + this->shape_name + "\n" +
                              "Perimeter: " + std::to_string(this->perimeter) + "\n" +
                              "Area: " + std::to_string(this->area) + "\n";
    if (this->colour != "")
    {
        description += "Colour: " + this->colour + "\n";
    }
    if (this->surface != "")
    {
        description += "Surface: " + this->surface + "\n";
    }

    return description;
}

AbstractShape factory_method(float h, float w, std::string colour, std::string surface)
{
    if (colour != "" || surface != "")
    {
        return ColourAndSurfaceRectangle(h, w, colour, surface);
    }
    return Rectangle(h, w);
}

test/BUILD文件

# test/BUILD

cc_test(
    name = "shape_test",
    size = "small",
    srcs = ["shape_test.cc"],
    deps = [
        "//main:shape",
        "@com_google_googletest//:gtest_main",
    ],
)

test/shape_test.cc文件

// test/shape_test.cc

#include <gtest/gtest.h>
#include "main/shape.h"
#include <string>

TEST(ColourAndSurfaceRectangleTest, ShapeTest)
{
    std::string expected_str = "Shape: Rectangle\n"
                               "Perimeter: 10\n"
                               "Area: 6\n"
                               "Colour: Blue\n"
                               "Surface: Stained\n";
    AbstractShape shape = factory_method(3, 2, "Blue", "Stained");
    EXPECT_EQ(shape.get_description(), expected_str);
}

错误信息 #

执行命令 bazel test --test_output=all //test:shape_test 后出现以下错误：

INFO: From Testing //test:shape_test:
==================== Test output for //test:shape_test:
dyld[37648]: symbol not found in flat namespace (__ZTV13AbstractShape)
================================================================================

问题 #

1. 这个symbol not found错误是什么含义？
2. macOS下使用Bazel/C++的用户有没有遇到过同类问题？

解答 #

错误含义 #

__ZTV13AbstractShape是C++编译器生成的虚函数表（vtable）符号，对应AbstractShape类的虚函数表。出现“symbol not found”说明程序运行时找不到这个虚函数表，核心原因是对象切片问题：
factory_method返回AbstractShape对象时，实际返回的是子类对象被“切片”后的基类对象——子类特有的数据和虚函数表信息被丢弃，调用虚函数get_description()时，无法找到正确的虚函数表，进而触发符号缺失错误。

macOS下同类问题 #

不少macOS用户用Bazel构建C项目时遇到过类似的虚函数表符号缺失问题，本质大多是C面向对象特性使用不当（比如对象切片），并非Bazel或macOS的特有bug。

修复方案 #

1. 将AbstractShape改为纯抽象类
   把get_description()声明为纯虚函数，确保基类无法被实例化，从根源避免对象切片：

   virtual std::string get_description() = 0; // 纯虚函数
   

2. 修改返回值类型为智能指针
   factory_method返回std::unique_ptr<AbstractShape>，这样不会发生对象切片，能完整保留子类的虚函数表信息：

   // shape.h中修改声明
   std::unique_ptr<AbstractShape> factory_method(float h, float w, std::string colour = "", std::string surface = "");
   
   // shape.cc中修改实现
   std::unique_ptr<AbstractShape> factory_method(float h, float w, std::string colour, std::string surface)
   {
       if (colour != "" || surface != "")
       {
           return std::make_unique<ColourAndSurfaceRectangle>(h, w, colour, surface);
       }
       return std::make_unique<Rectangle>(h, w);
   }
   

3. 调整测试代码
   使用指针调用虚函数，同时注意std::to_string会输出浮点数的完整格式（比如10会变成10.000000），需调整预期字符串匹配实际输出：

   TEST(ColourAndSurfaceRectangleTest, ShapeTest)
   {
       std::string expected_str = "Shape:Rectangle\n"
                                  "Perimeter: 10.000000\n"
                                  "Area: 6.000000\n"
                                  "Colour: Blue\n"
                                  "Surface: Stained\n";
       auto shape = factory_method(3, 2, "Blue", "Stained");
       EXPECT_EQ(shape->get_description(), expected_str);
   }
   

内容的提问来源于stack exchange，提问作者mndl