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TryStrToBool

function TryStrToBool(const S: string; out Value: Boolean): Boolean;

Example​

procedure ScriptEvent(var Value: variant);
var
  Parsed: Boolean;
begin
  if TryStrToBool(VarToStr(Value), Parsed) then
    Value := Parsed
  else
    Value := False;
end;

Usage​

TryStrToBool attempts to convert numeric or Boolean text and reports whether the output value is valid.

Parameters

NameTypeDescription
SstringNumeric or named Boolean text to test. Numeric zero is false; non-zero numeric input is true; names are case-insensitive.
ValueBoolean (out)Receives the converted Boolean only when the function returns True. Its content is not a valid result after failure.

Returns

True when numeric parsing or configured-name matching succeeds; False otherwise. This return is conversion status, not the parsed Boolean itself.

Behaviour

The two Boolean outcomes therefore require two values: the function result says whether conversion succeeded, while the out parameter contains true or false data. A valid false input such as 0 or False returns function result True and output value False.

Errors

Unrecognised text returns False rather than raising. Unexpected allocation or runtime errors are not converted to a false status.

Usage notes

Velox uses short-circuit Boolean evaluation, so if TryStrToBool(S, B) and B then ... does not read B after a failed conversion. A nested if or separate statements can make the success-before-value rule clearer.

Additional Technical Info​

TryStrToBool attempts the same numeric-first and named-Boolean conversion as StrToBool, but returns a success flag instead of raising for unrecognised text. Read the Value output only when the function returns True.

The example is source-reviewed and is not executed by the documentation workflow.

Implementation

The Velox SysUtils scripting adapter calls Delphi System.SysUtils.TryStrToBool. The terminal first calls the global-format overload of TryStrToFloat. Successful numeric conversion assigns Value := Number <> 0. If numeric conversion fails, it lazily verifies the process-wide TrueBoolStrs/FalseBoolStrs arrays and compares S against their values without regard to case.

Edge cases and quirks

  • Numeric parsing happens before name matching and uses shared Delphi FormatSettings, not Velox's system-locale StrToFloat wrapper.
  • The installed floating terminal recognises NAN, INF, +INF and -INF. These are successful numeric conversions and currently yield output True through the non-zero comparison.
  • Named matching ignores case but does not trim the input. Added whitespace can turn a named Boolean into a failed conversion.
  • If both configured arrays contain the same spelling, the true-array check wins because it is performed first.
  • On complete failure the implementation does not assign a meaningful Value. Free Pascal explicitly documents the output as undetermined in this case; the same safe-use rule applies to the Delphi/Velox path.
  • Empty Boolean-name arrays can be initialised lazily with the default True and False strings.

Side effects

The first non-numeric call can initialise the process-wide Boolean string arrays. No Velox data is changed except through the caller's deliberate use of the out value.

Performance and concurrency

Linear in input and configured-name lengths. The routine reads global format settings and can initialise shared name arrays, so concurrent host mutation of those globals can affect conversion.

Related entries

  • StrToBool returns the parsed value and raises on failure.
  • StrToBoolDef replaces failure with a supplied Boolean.
  • StrToFloat documents Velox's separate system-locale floating conversion.

External references

Created 2026-07-15