Add eckit::format_to

Author: geier1993

src/eckit/format/Format.h

@@ -15,6 +15,9 @@
 #include <fmt/ranges.h>
 #include <fmt/std.h>
 
+#include <type_traits>
+#include <iterator>
+
 #define ENABLE_FORMAT(typ) \
     template <>            \
     struct fmt::formatter<typ> : fmt::ostream_formatter {}
@@ -49,4 +52,28 @@ std::string format(StringType&& formatString, Args&&... args) {
 }
 
 
+/// Format a string with compile time checks to an output iterator
+/// @param outputIt output iterator to write to
+/// @param formatString to use, see: <https://fmt.dev/11.1/syntax/> for description of syntax.
+///        Must be known at compiletime
+/// @param ... args to be upplied into formatString
+/// @throws fmt::format_error
+template <typename OutputIt, typename StringType, typename... Args, std::enable_if_t<!std::is_base_of_v<std::ostream, std::decay_t<OutputIt>>, bool> = true>
+void format_to(OutputIt&& outputIt, StringType&& formatString, Args&&... args) {
+    fmt::format_to(std::forward<OutputIt>(outputIt), fmt::runtime(std::forward<StringType>(formatString)), std::forward<Args>(args)...);
+}
+
+/// Format a string with compile time checks to an ostream
+/// @param os ostream to write to
+/// @param formatString to use, see: <https://fmt.dev/11.1/syntax/> for description of syntax.
+///        Must be known at compiletime
+/// @param ... args to be upplied into formatString
+/// @throws fmt::format_error
+template <typename StringType, typename... Args>
+void format_to(std::ostream& os, StringType&& formatString, Args&&... args) {
+    // Have to either use `const char&` or `const wchar_t&` here 
+    fmt::format_to(std::ostream_iterator<const char&>(os), fmt::runtime(std::forward<StringType>(formatString)), std::forward<Args>(args)...);
+}
+
+
 }  // namespace eckit

tests/format/CMakeLists.txt

@@ -2,3 +2,7 @@ ecbuild_add_test( TARGET      eckit_test_format
                   SOURCES     test_format.cc
                   LIBS        eckit )
 
+ecbuild_add_test( TARGET      eckit_test_format_to
+                  SOURCES     test_format_to.cc
+                  LIBS        eckit )
+

tests/format/test_format.cc

@@ -55,7 +55,6 @@ CASE("ENABLE_FORMAT CodeLocation") {
     std::cout << eckit::format("{}", Here());
 }
 
-
 int main(int argc, char** argv) {
     return eckit::testing::run_tests(argc, argv);
 }

tests/format/test_format_to.cc

@@ -0,0 +1,49 @@
+/*
+ * (C) Copyright 2025- ECMWF.
+ *
+ * This software is licensed under the terms of the Apache Licence Version 2.0
+ * which can be obtained at http://www.apache.org/licenses/LICENSE-2.0.
+ * In applying this licence, ECMWF does not waive the privileges and immunities
+ * granted to it by virtue of its status as an intergovernmental organisation nor
+ * does it submit to any jurisdiction.
+ */
+#include <eckit/testing/Test.h>
+
+#include "eckit/format/Format.h"
+#include "eckit/log/Log.h"
+#include "eckit/log/OStreamTarget.h"
+#include "eckit/log/Channel.h"
+
+#include <sstream>
+
+CASE("Format to output iterator") {
+    {
+        auto output = fmt::memory_buffer();
+        eckit::format_to(std::back_inserter(output), "Hello {} {} {}", "through", "a", "stream");
+        EXPECT_EQUAL(std::string(output.data(), output.size()), std::string("Hello through a stream"));
+    }
+    {
+        std::vector<char> output;
+        eckit::format_to(std::back_inserter(output), "Hello {} {} {}", "through", "a", "stream");
+        EXPECT_EQUAL(std::string(output.data(), output.size()), std::string("Hello through a stream"));
+    }
+}
+
+CASE("Format to streams") {
+    std::ostringstream oss;
+    eckit::format_to(oss, "Hello {} {} {}", "through", "a", "stream");
+    EXPECT_EQUAL(oss.str(), std::string("Hello through a stream"));
+}
+
+CASE("Format to eckit channel") {
+    std::ostringstream oss;
+    eckit::Channel ch{new eckit::OStreamTarget{oss}};
+    eckit::format_to(ch, "Hello {} {} {}", "through", "a", "stream");
+    ch.flush();
+    EXPECT_EQUAL(oss.str(), std::string("Hello through a stream"));
+}
+
+
+int main(int argc, char** argv) {
+    return eckit::testing::run_tests(argc, argv);
+}