Adding take_last

README.md

@@ -155,6 +155,7 @@ The add commands return a pointer to an internally stored `Option`. If you set t
 * `->envname(name)`: Gets the value from the environment if present and not passed on the command line.
 * `->group(name)`: The help group to put the option in. No effect for positional options. Defaults to `"Options"`. `"Hidden"` will not show up in the help print.
 * `->ignore_case()`: Ignore the case on the command line (also works on subcommands, does not affect arguments).
+* `->take_last()`: Only take the last option/flag given on the command line, automatically true for bool flags
 * `->check(CLI::ExistingFile)`: Requires that the file exists if given.
 * `->check(CLI::ExistingDirectory)`: Requires that the directory exists.
 * `->check(CLI::NonexistentPath)`: Requires that the path does not exist.

include/CLI/App.hpp

@@ -351,22 +351,23 @@ class App {
         return opt;
     }
 
-    /// Bool version
+    /// Bool version - defaults to allowing multiple passings, but can be forced to one if `take_last(false)` is used.
     template <typename T, enable_if_t<is_bool<T>::value, detail::enabler> = detail::dummy>
     Option *add_flag(std::string name,
                      T &count, ///< A varaible holding true if passed
                      std::string description = "") {
 
         count = false;
-        CLI::callback_t fun = [&count](CLI::results_t) {
+        CLI::callback_t fun = [&count](CLI::results_t res) {
             count = true;
-            return true;
+            return res.size() == 1;
         };
 
         Option *opt = add_option(name, fun, description, false);
         if(opt->get_positional())
             throw IncorrectConstruction("Flags cannot be positional");
         opt->set_custom_option("", 0);
+        opt->take_last();
         return opt;
     }
 

include/CLI/Option.hpp

@@ -74,6 +74,9 @@ class Option {
     /// The number of expected values, 0 for flag, -1 for unlimited vector
     int expected_{1};
 
+    /// Only take the last argument (requires `expected_ == 1`)
+    bool last_{false};
+
     /// A private setting to allow args to not be able to accept incorrect expected values
     bool changeable_{false};
 
@@ -155,10 +158,20 @@ class Option {
             throw IncorrectConstruction("Cannot make a flag take arguments!");
         else if(!changeable_)
             throw IncorrectConstruction("You can only change the expected arguments for vectors");
+        else if(last_)
+            throw IncorrectConstruction("You can't change expected arguments after you've set take_last!");
         expected_ = value;
         return this;
     }
 
+    /// Take the last argument if given multiple times
+    Option *take_last(bool value = true) {
+        if(expected_ != 0 && expected_ != 1)
+            throw IncorrectConstruction("take_last only works for flags and single value options!");
+        last_ = value;
+        return this;
+    }
+
     /// Adds a validator
     Option *check(std::function<bool(std::string)> validator) {
 
@@ -243,6 +256,9 @@ class Option {
     /// The number of arguments the option expects
     int get_expected() const { return expected_; }
 
+    /// The status of the take last flag
+    bool get_take_last() const { return last_; }
+
     /// True if this has a default value
     int get_default() const { return default_; }
 
@@ -340,7 +356,16 @@ class Option {
 
     /// Process the callback
     void run_callback() const {
-        if(!callback_(results_))
+        bool result;
+        // If take_last, only operate on the final item
+        if(last_) {
+            results_t partial_result = {results_.back()};
+            result = !callback_(partial_result);
+        } else {
+            result = !callback_(results_);
+        }
+
+        if(result)
             throw ConversionError(get_name() + "=" + detail::join(results_));
         if(!validators_.empty()) {
             for(const std::string &result : results_)
@@ -407,7 +432,7 @@ class Option {
             return std::find(std::begin(lnames_), std::end(lnames_), name) != std::end(lnames_);
     }
 
-    /// Puts a result at position r
+    /// Puts a result at the end, unless last_ is set, in which case it just keeps the last one
     void add_result(std::string s) {
         results_.push_back(s);
         callback_run_ = false;

tests/AppTest.cpp

@@ -167,6 +167,20 @@ TEST_F(TApp, BoolAndIntFlags) {
     EXPECT_EQ((unsigned int)2, uflag);
 }
 
+TEST_F(TApp, BoolOnlyFlag) {
+    bool bflag;
+    app.add_flag("-b", bflag)->take_last(false);
+
+    args = {"-b"};
+    EXPECT_NO_THROW(run());
+    EXPECT_TRUE(bflag);
+
+    app.reset();
+
+    args = {"-b", "-b"};
+    EXPECT_THROW(run(), CLI::ConversionError);
+}
+
 TEST_F(TApp, ShortOpts) {
 
     unsigned long long funnyint;
@@ -204,6 +218,18 @@ TEST_F(TApp, DefaultOpts) {
     EXPECT_EQ("9", s);
 }
 
+TEST_F(TApp, TakeLastOpt) {
+
+    std::string str;
+    app.add_option("--str", str)->take_last();
+
+    args = {"--str=one", "--str=two"};
+
+    run();
+
+    EXPECT_EQ(str, "two");
+}
+
 TEST_F(TApp, EnumTest) {
     enum Level : std::int32_t { High, Medium, Low };
     Level level = Level::Low;