ROS: learning

ROS: learning"/>

ROS: learning

错误产生

在阅读《ROS机器人开发实践》这本书的3.6时，编译cmake工程出现了如下报错：

CMake Error at learning_communication/CMakeLists.txt:125 (add_dependencies):The dependency target "learning_communication_generate_messages_cpp" oftarget "talker" does not exist.-- Generating done
-- Build files have been written to: /../../catkin_ws/build
Makefile:556: recipe for target 'cmake_check_build_system' failed
make: *** [cmake_check_build_system] Error 1
Invoking "make cmake_check_build_system" failed

错误产生原因

通过百度找到了如下的解决方案，即修改CMakeLists.txt首行的版本号，将cmake版本降为2.8.3。因此确定是cmake版本问题，那么cmake的版本更新中究竟做了哪些改动呢？这个可以在cmake官方文档中查找到，描述如下：

Error on non-existent dependency in add_dependencies.
CMake 2.8.12 and lower silently ignored non-existent dependencies listed in the add_dependencies() command.
The OLD behavior for this policy is to silently ignore non-existent dependencies. The NEW behavior for this policy is to report an error if non-existent dependencies are listed in the add_dependencies() command.
This policy was introduced in CMake version 3.0. CMake version 3.0.2 warns when the policy is not set and uses OLD behavior. Use the cmake_policy command to set it to OLD or NEW explicitly.

也就是说，在2.8.12以下的版本中，cmake的add_dependencies命令会忽略掉不存在的依赖，继续编译，而升级后的cmake遇到同样的问题采取的默认行为是报错（可通过cmake_policy修改），因此就产生了ROS编译中的问题。

为什么依赖不存在呢？首先要明白这句命令的作用。看书中原话可知，这行命令是添加程序功能包动态产生的消息代码，而在本次工程中并未使用到此功能。

解决方法

以下方法任选一种即可：

1. 降低cmake版本：即将cmake_minimum_required(VERSION 3.0.2)改为cmake_minimum_required(VERSION 2.8.3)
2. 注释掉add_dependencies(talker ${PROJECT_NAME}_generate_messages_cpp)和add_dependencies(talker ${PROJECT_NAME}_generate_messages_cpp)
3. 通过cmake_policy修改默认行为