Reading Multiple Objects from Stream
What is the best way to read multiple objects from a stream?
Reading multiple objects from a stream can be efficiently handled using a loop. This approach allows you to continuously extract data until the end of the stream is reached or an error occurs.
Example with a Simple Class
Consider a simple Character class and reading multiple Character objects from a stream:
#include <iostream>
#include <sstream>
#include <vector>
class Character {
public:
std::string Name;
int Level;
bool isAlive;
Character(std::istringstream& stream) {
stream >> Name >> Level >> isAlive;
}
void Log() const {
std::cout << Name << " - Level " << Level
<< (isAlive ? " (Alive)" : " (Dead)")
<< '\n';
}
};
int main() {
std::istringstream input{
"Legolas 80 1 Gimli 70 1 Aragorn 60 0"};
std::vector<Character> party;
while (input) {
Character c(input);
// Check if the extraction was successful
if (input) {
party.emplace_back(std::move(c));
}
}
for (const auto& member : party) {
member.Log();
}
}Legolas - Level 80 (Alive)
Gimli - Level 70 (Alive)
Aragorn - Level 60 (Dead)This example reads characters from the input stream until no more data is available.
Handling Errors and End of Stream
To make the process more robust, you should handle potential errors and check the state of the stream correctly. Here's an improved version:
#include <iostream>
#include <sstream>
#include <vector>
class Character {
public:
std::string Name;
int Level;
bool isAlive;
bool readFromStream(std::istringstream& stream) {
int alive;
if (stream >> Name >> Level >> alive) {
isAlive = static_cast<bool>(alive);
return true;
}
return false;
}
void Log() const {
std::cout << Name << " - Level " << Level
<< (isAlive ? " (Alive)" : " (Dead)")
<< '\n';
}
};
int main() {
std::istringstream input{
"Legolas 80 1 Gimli 70 1 Aragorn 60 0"};
std::vector<Character> party;
Character character;
while (character.readFromStream(input)) {
party.push_back(character);
}
for (const auto& member : party) {
member.Log();
}
}Legolas - Level 80 (Alive)
Gimli - Level 70 (Alive)
Aragorn - Level 60 (Dead)In this version, the readFromStream() method returns false when it fails to read more data, effectively ending the loop.
Using std::getline()
If your objects are defined across multiple lines or use custom delimiters, consider using std::getline():
#include <iostream>
#include <sstream>
#include <vector>
class Character {
public:
std::string Name;
int Level;
bool isAlive;
Character(const std::string& line) {
std::istringstream stream(line);
stream >> Name >> Level >> isAlive;
}
void Log() const {
std::cout << Name << " - Level " << Level
<< (isAlive ? " (Alive)" : " (Dead)")
<< '\n';
}
};
int main() {
std::istringstream input{
"Legolas 80 1\nGimli 70 1\nAragorn 60 0"};
std::vector<Character> party;
std::string line;
while (std::getline(input, line)) {
party.emplace_back(line);
}
for (const auto& member : party) {
member.Log();
}
}Legolas - Level 80 (Alive)
Gimli - Level 70 (Alive)
Aragorn - Level 60 (Dead)This approach allows for more complex input formats, making your code adaptable to various data sources.
Reading multiple objects from a stream involves using loops and proper state checking to ensure data is read correctly and efficiently.
Input Streams
A detailed introduction to C++ Input Streams using std::cin and std::istringstream. Starting from the basics and progressing up to advanced use cases including creating collections of custom objects from our streams.