#include 
...

// parse a Steam ID in "SteamID3" format.
SteamID steamid( "[U:1:108998443]", SteamID::Formats::STEAMID3 );

// print it in SteamID32 format
std::cout << steamid[ SteamID::Formats::STEAMID32 ];

// (prints "STEAM_1:1:54499221")