c++ Inset and read Map element in protobuf3 - c++11

Question

Here is the .proto which have one map:
map.proto
syntax = "proto3";
package demo;
message Person {
map<string, int32> family_list = 4;
}
Now I insert key and value from the Write_impl.cc
auto map = test.mutable_family_list();
string key = "faimly";
int val =20;
(*map)[key] = val;
std::cout<<"map = "<<(*map)[key]<<std::endl;
Below I read the value of key family in read_impl.cc
auto test = demo::Person::default_instance();
auto map = test.mutable_family_list();
std::cout<<"map = "<<(*map)["faimly"]<<std::endl;
Problem: I get 0 when reading the value of key "family"

Answer 1

You are using demo::Person::default_instance() which doesn't have the value you've stored earlier. It contains the default one.
You are using the subscript operator [] that doesn't throw an exception if the key is not found but the at() method does. You should use the at() method.
Here's an example of serialization and deserialization:
int main()
{
// Serialization
demo::Person sPerson;
const auto mutable_family_list = sPerson.mutable_family_list();
mutable_family_list->insert( { "abc", 42 } );
std::cout << mutable_family_list->at( "abc" ) << '\n';
const auto serialized = sPerson.SerializeAsString();
// Deserialization
demo::Person dPerson;
if ( !dPerson.ParseFromString( serialized ) )
{
std::cerr << "Deserialization failed!\n";
return -1;
}
const auto family_list = dPerson.family_list();
std::cout << family_list.at("abc") << '\n';
return 0;
}
Output:
42
42
Alternatively, you can find() first and then use the value like this:
const auto it = family_list.find( "abc" );
if ( it != family_list.end() )
{
std::cout << it->second << '\n';
}