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57 lines
2.3 KiB
Markdown
57 lines
2.3 KiB
Markdown
# Validators
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{% hint style='info' %}
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Improved in CLI11 1.6
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{% endhint %}
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There are two forms of validators:
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* `transform` validators: mutating
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* `check` validators: non-mutating (recommended unless the parsed string must be mutated)
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A transform validator comes in one form, a function with the signature `std::string(std::string)`.
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The function will take a string and return the modified version of the string. If there is an error,
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the function should throw a `CLI::ValidationError` with the appropriate reason as a message.
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However, `check` validators come in two forms; either a simple function with the const version of the
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above signature, `std::string(const std::string &)`, or a subclass of `struct CLI::Validator`. This
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structure has two members that a user should set; one (`func`) is the function to add to the Option
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(exactly matching the above function signature, since it will become that function), and the other is
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`tname`, and is the type name to set on the Option (unless empty, in which case the typename will be
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left unchanged).
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Validators can be combined with `&` and `|`, and they have an `operator()` so that you can call them
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as if they were a function. In CLI11, const static versions of the validators are provided so that
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the user does not have to call a constructor also.
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An example of a custom validator:
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```cpp
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struct LowerCaseValidator : public Validator {
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LowerCaseValidator() {
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tname = "LOWER";
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func = [](const std::string &str) {
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if(CLI::detail::to_lower(str) != str)
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return std::string("String is not lower case");
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else
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return std::string();
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};
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}
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};
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const static LowerCaseValidator Lowercase;
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```
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If you were not interested in the extra features of Validator, you could simply pass the lambda function above to the `->check()` method of `Option`.
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The built-in validators for CLI11 are:
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| Validator | Description |
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| `ExistingFile` | Check for existing file (returns error message if check fails) |
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| `ExistingDirectory` | Check for an existing directory (returns error message if check fails) |
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| `ExistingPath` | Check for an existing path |
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| `NonexistentPath` | Check for an non-existing path |
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| `Range(min=0, max)` | Produce a range (factory). Min and max are inclusive. |
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