CmakeLists.txt

PROJECT(ex)
CMAKE_MINIMUM_REQUIRED(VERSION 2.8)
INCLUDE(FindProtobuf)
FIND_PACKAGE(Protobuf REQUIRED)

FILE(GLOB ProtoFiles "${CMAKE_CURRENT_SOURCE_DIR}/*.proto")
PROTOBUF_GENERATE_CPP(ProtoSources ProtoHeaders ${ProtoFiles})
FILE(GLOB SRC_EX *.cc)

INCLUDE_DIRECTORIES(${CMAKE_SOURCE_DIR})
INCLUDE_DIRECTORIES(${CMAKE_CURRENT_BINARY_DIR})
ADD_EXECUTABLE(ex ${SRC_EX} ${ProtoSources})
TARGET_LINK_LIBRARIES(ex ${PROTOBUF_LIBRARIES})

main.cc

#include <iostream>
#include "person.pb.h"

int main()
{
    Person p, p2;
    std::string result;

    p.set_name("Chobie");
    p.SerializeToString(&result);
    std::string const& s = static_cast<const std::string &>(result);

    p2.ParseFromString(s);
    std::cout << p2.name() << std::endl;
    std::cout << p2.phone().number() << std::endl;
    return 0;
}

person.proto

message Person
{
    optional string name = 1;
    message Phone
    {
        optional string number = 1 [default = "12345"];
    }
    optional Phone phone = 2;
}

mkdir build && cd build && cmake .. && make
./ex

macbook% ./ex
Chobie
12345