How do you safely parse a CSV line into typed values in C++?

A strong answer

Start with the data: a line like 42,Ada,30 is just text. You need three steps: split on commas, convert each slice to the right type, and validate the result.

For splitting, use std::string_view over the original line — no copy. Walk the view with a position index, find the next comma, substr the field, advance past the comma. This is O(n) on the line length with zero allocations.

For conversion, use std::from_chars (C++17). It takes a pointer range and an output reference, returns {ptr, ec}. Check ec == std::errc{} for success, and ptr == end to ensure the whole field was a valid number — from_chars stops at the first non-digit, so 123abc would parse as 123 with ptr pointing at a. The ptr == end check rejects that. from_chars is noexcept, has no locale dependence, and does no heap allocation. It is strictly safer and faster than std::stoi/std::stod, which throw on failure, skip whitespace silently, and can allocate.

Empty fields: substr returns an empty view. from_chars on empty fails with errc::invalid_argument, which is correct — an empty string is not a valid integer. If your format allows optional fields, check for empty before calling from_chars and treat it as a missing value (e.g., std::optional<int>).

Malformed lines (wrong field count, non-numeric age): the parser returns std::nullopt or a boolean false. The caller decides: skip, log, or abort. Never let a partially parsed struct escape into your data structures.

What the interviewer is listening for

• string_view + position index for zero-copy splitting.
• from_chars for conversion, with the ptr == end full-consumption check.
• Explicit handling of empty fields and malformed input.
• Why from_chars beats stoi: noexcept, no locale, no allocation, exact consumption check.
• Return type that forces the caller to handle failure (optional, expected, or bool+out-param).

Common mistakes

• Using stoi/stod and catching exceptions — slow, locale-sensitive, silent whitespace skipping.
• Forgetting the ptr == end check, so 123abc becomes 123 without warning.
• Modifying the original string (e.g., strtok) instead of using string_view.
• Not advancing the position past the delimiter, causing the next field to start with a comma.
• Letting a partially parsed object leak into a container — always validate before insert.