C ++地图 <string, vector<pair<string, string> &gt;&gt;&gt;：向空向量添加映射？

Question

I'm very new to C++ container templates. I have a collection of records . Each record has a unique name, and a list of field/value pairs. The records will be accessed by name. The order of the field/value pairs is important. Hence I've designed it as follows:

typedef string      Typecode;
typedef string      Fieldname;
typedef string      Fieldvalue;
typedef vector<pair<Fieldname, Fieldvalue> >  Field_value_pairs;
typedef map<Typecode, Field_value_pairs>      Record_map;

Record_map          records;

I want to define a method add_record(Typecode) that will add an entry to records with a key of type Typecode and an empty Field_value_pairs vector. (At some point later on I will add some or all of the field/value pairs.) But I can't seem to figure out what map<> and vector<> methods to use.

I think I want to use operator= , as in records["foo_record"] = . But what should I assign as the value, to create an "empty vector of pairs"?

Answer 1

You should assign as:

records["foo_record"] = vector<pair<Fieldname, FieldValue> >();

std::vector's default constructor will create an empty vector, and then you can add new values to it using

records["foo_record"].push_back(pair<Fieldname, FieldValue>("name", "value"));

Answer 2

Default construct a Field_value_pairs object and assign it to the new map entry.

void add_record( Record_map& records, Typecode const& code )
{
    records[code] = Field_value_pairs();
}

Beware that this will overwrite any existing entry for that Typecode . If you want to conditionally add a Typecode only if one doesn't already exist, use map::find to determine whether the entry exists.

void add_record( Record_map& records, Typecode const& code )
{
    if( records.find( code ) == records.end() ) {
        records[code] = Field_value_pairs();
    } else {
        // entry exists, do something else
    }
}