std::optional
Introduced in C++17, it is used to represent a type whose content is not always present.
Present in the header, optional.
std::optional can either contain a value or be empty.
If you ever what to return a status along with the value of interest, use optional instead of giving (by return or modifying arguments).
Without optional, the code would look like this:
bool ReadFile(const string& filePath, string& fileContent)
{
fstream fileStream(filePath);
if (fileStream)
{
// open file and copy the content
fileContent = content;
return true;
}
fileContent = string();
return false;
}
void client()
{
string fileData;
if (ReadFile("config.txt", fileData))
{
// use fileData
}
else
{
// do the default operations
}
}
Although the above code works as intented, C++17 provides a more pragmatic of achieving the similar functionality.
std::optional<std::string> ReadFile(const string& filePath)
{
fstream fileStream(filePath);
if (fileStream)
{
string fileContent;
// open file and copy the content
return fileContent;
}
return {};
}
void client()
{
std::optional<std::string> fileData = ReadFile("config.txt");
if (fileData.has_value()) // or if (fileData) - since it has a boolean operator
{
// use fileData
}
else
{
// do the default operations
}
string data = fileData.value_or(string{"default"});
}
The validity of the optional can be checked by has_value function, which return a bool whether it contains a value or not.
The data can be extracted with value function. However, if the to assign a default option when there is no value present, use value_or function, which takes as an argument the default value to be returned. Fully functional code is available at this gist.