C++ formatted input must match value - c++

Question

I'm reading a file with C++; the file looks like:
tag1 2345
tag2 3425
tag3 3457
I would like to have something like
input>>must_be("tag1")>>var1>>must_be("tag2")>>var2>>must_be("tag3")>>var3;
Where everything blows up if what's being taken in doesn't match the argument of must_be() and, when done, var1=2345, var2=3425, var3=3457.
Is there a standard way of doing this? (Hopefully where "tag1" need not necessarily be a string, but this is not a requirement.) fscanf from C made it quite easy.
Thanks!
To clarify, each >> reads in one whitespace-delimited set of characters from input. I want to match some of the in-coming blocks of characters (tagX) against strings or data I have specified.

Answer 1

You need to implement operator>> for your class. Something like this :
#include <string>
#include <iostream>
#include <fstream>
#include <sstream>
struct A
{
A(const int tag_):tag(tag_),v(0){}
int tag;
int v;
};
#define ASSERT_CHECK( chk, err ) \
if ( !( chk ) ) \
throw std::string(err);
std::istream& operator>>( std::istream & is, A &a )
{
std::string tag;
is >> tag;
ASSERT_CHECK( tag.size() == 4, "tag size" );
std::stringstream ss(std::string(tag.begin()+3,tag.end()));
int tagVal;
ss >> tagVal;
std::cout<<"tag="<<tagVal<<" a.tag="<<a.tag<<std::endl;
ASSERT_CHECK( a.tag == tagVal,"tag value" );
is >> a.v;
return is;
}
int main() {
A a1(1);
A a2(2);
A a3(4);
try{
std::fstream f("in.txt" );
f >> a1 >> a2 >> a3;
}
catch(const std::string &e)
{
std::cout<<e<<std::endl;
}
std::cout<<"a1.v="<<a1.v<<std::endl;
std::cout<<"a2.v="<<a2.v<<std::endl;
std::cout<<"a3.v="<<a3.v<<std::endl;
}
Take a note that for wrong tag value, an exception will be thrown (meaning the tag much match).

Answer 2

Can't you read it line by line, and matching tags for each line? If the tag doesn't match what you expect you just skip the line and move on to the next.
Something like this:
const char *tags[] = {
"tag1",
"tag2",
"tag3",
};
int current_tag = 0; // tag1
const int tag_count = 3; // number of entries in the tags array
std::map<std::string, int> values;
std::string line;
while (current_tag < tag_count && std::getline(input, line))
{
std::istringstream is(line);
std::string tag;
int value;
is >> tag >> value;
if (tag == tags[current_tag])
values[tag] = value;
// else skip line (print error message perhaps?)
current_tag++;
}