The Issue with the Scope of Variable for lambdas in Structured Binding Declarations
Structured bindings in C++ have introduced a convenient way to bind multiple variables to the elements of a structured entity. However, there is a specific issue related to the scope of variables declared using structured bindings, such as the variable startingBalance in the given example.
1. Introduction
In modern C++ programming, structured bindings (since c++17) provide a concise syntax to unpack and bind multiple elements from a structured entity, such as tuples or arrays, to individual variables. While they offer improved readability and ease of use, there are certain limitations associated with structured bindings that developers need to be aware of.
2. Understanding Structured Bindings
Structured bindings allow you to declare multiple variables and bind them to the elements of a structured entity in a single statement. For example, the code snippet provided demonstrates a structured binding declaration:
auto [startingBalance, startingAssetBalance] = getSpotBalances(quoteAsset, baseAsset, api);
In this case, the function getSpotBalances returns a std::pair or a similar structured entity with two elements, and the structured binding declaration assigns those elements to the variables startingBalance and startingAssetBalance.
3. The Issue with Capturing Structured Bindings in Lambdas
One of the limitations of structured bindings is that they cannot be captured by reference in a lambda function. In the given example, if you attempt to capture the references to variables startingBalance and startingAssetBalance in a lambda, you will encounter a compilation error.
// Incorrect code - Compilation error
auto lambda = [&startingBalance, &startingAssetBalance]() {
// Your lambda code here
};
This limitation arises because the variables declared in structured bindings are not true variables; they act more like references to the elements of the structured entity. As a result, they cannot be captured by reference in a lambda.
4. Resolving the Issue
To overcome the issue of capturing structured bindings in lambdas, one approach is to use individual auto variables instead of structured bindings. By assigning each element of the structured entity to a separate auto variable, you can capture them in a lambda function successfully. Here's an updated version of the code snippet:
auto balances = getSpotBalances(quoteAsset, baseAsset, api);
double startingBalance = balances.first;
double startingAssetBalance = balances.second;
auto lambda = [&startingBalance, &startingAssetBalance]() {
// Your lambda code here
};
In this modified code, we retrieve the elements from getSpotBalances using a single auto variable named balances. Then, we assign the respective references to separate auto variables startingBalance and startingAssetBalance. These individual references can now be captured in the lambda without any compilation errors.
5. Conclusion
Structured bindings provide a convenient way to unpack and bind elements from a structured entity in C++. However, capturing structured bindings in lambdas is not supported in C++ standards up to C++17. To work around this limitation, you can use individual auto variables to capture the references and successfully utilize them in lambda functions.
Remember to adapt your code according to the C++ standard and choose the appropriate method based on your specific requirements and constraints.
6. FAQs
Q1. Can structured bindings be captured by value in lambdas? Yes, structured bindings can be captured by value in lambdas. The limitations of capturing structured bindings apply only to reference captures.
Q2. Are there any other alternatives to structured bindings for unpacking structured entities? Prior to the introduction of structured bindings, developers used functions like std::tie or directly accessed elements by index or key. These alternatives are still valid and can be used depending on the situation.
Q3. Does the limitation of capturing structured bindings apply to all types of structured entities? Yes, the limitation of capturing structured bindings applies to all types of structured entities, including tuples, arrays, and other similar structures.