Store specifier strings in runtime format_template for more detailed exception messages

1 files changed, 34 insertions, 18 deletions

diff --git a/include/cxxformat/runtime.hpp b/include/cxxformat/runtime.hpp
index a33264e..85c6ecf 100644
--- a/include/cxxformat/runtime.hpp
+++ b/include/cxxformat/runtime.hpp
@@ -11,6 +11,13 @@
 namespace format {
 	namespace {
 	namespace run_time {
+		struct format_specifier {
+			format::format_specifier base;
+			std::string_view source;
+
+			constexpr format_specifier(format::format_specifier base, std::string_view source) noexcept : base{base}, source{source} {}
+		};
+
 		class format_template {
 			using Part = std::variant<std::string_view, format_specifier>;
 			std::string formatString;
@@ -19,8 +26,8 @@ namespace format {
 
 			template<typename Out>
 			struct argument_formatter {
-				virtual void format(const Out& out, format_specifier spec, optional_int<std::size_t> minWidth, optional_int<std::size_t> precision) = 0;
-				virtual void conversionSupported(format_specifier spec) = 0;
+				virtual void format(const Out& out, format::format_specifier spec, optional_int<std::size_t> minWidth, optional_int<std::size_t> precision) = 0;
+				virtual void conversionSupported(format::format_specifier spec) = 0;
 				virtual ~argument_formatter() = default;
 			};
 
@@ -33,12 +40,12 @@ namespace format {
 			public:
 				argument_formatter_for(Arg&& arg) : arg{std::forward<Arg>(arg)} {}
 
-				void format(const Out& out, format_specifier spec, optional_int<std::size_t> minWidth, optional_int<std::size_t> precision) override
+				void format(const Out& out, format::format_specifier spec, optional_int<std::size_t> minWidth, optional_int<std::size_t> precision) override
 				{
 					formatter::format(out, arg, spec, minWidth, precision);
 				}
 
-				void conversionSupported(format_specifier spec) override
+				void conversionSupported(format::format_specifier spec) override
 				{
 					formatter::conversionSupported(spec);
 				}
@@ -78,28 +85,33 @@ namespace format {
 				{
 					if (std::holds_alternative<format_specifier>(part))
 					{
-						const auto spec = std::get<format_specifier>(part);
-						if (spec.flags.widthAsArg)
-						{
-							if (!indexArgs[*spec.minWidth])
+						const auto [spec, source] = std::get<format_specifier>(part);
+						try {
+							if (spec.flags.widthAsArg)
 							{
-								throw std::invalid_argument{"Width argument must be an unsigned integral at most as big as std::size_t"};
+								if (!indexArgs[*spec.minWidth])
+								{
+									throw std::invalid_argument{"Width argument must be an unsigned integral at most as big as std::size_t"};
+								}
 							}
-						}
-						if (spec.flags.precisionAsArg)
-						{
-							if (!indexArgs[*spec.precision])
+							if (spec.flags.precisionAsArg)
 							{
-								throw std::invalid_argument{"Precision argument must be an unsigned integral at most as big as std::size_t"};
+								if (!indexArgs[*spec.precision])
+								{
+									throw std::invalid_argument{"Precision argument must be an unsigned integral at most as big as std::size_t"};
+								}
 							}
+							argFormatters[spec.argIndex]->conversionSupported(spec);
+						} catch (const std::exception& e) {
+							throw std::runtime_error{"%s; when checking format specifier “%s”"_format(e.what(), source)};
 						}
-						argFormatters[spec.argIndex]->conversionSupported(spec);
 					}
 				}
 
 				const overloaded_callable visitor{
-					[&out, &indexArgs, &argFormatters](format_specifier spec)
+					[&out, &indexArgs, &argFormatters](const format_specifier& fullSpec)
 					{
+						const auto [spec, source] = fullSpec;
 						optional_int<std::size_t> minWidth = spec.minWidth;
 						if (spec.flags.widthAsArg)
 						{
@@ -112,7 +124,11 @@ namespace format {
 							precision = indexArgs[*precision];
 							assert(precision);
 						}
-						return argFormatters[spec.argIndex]->format(out, spec, minWidth, precision);
+						try {
+							return argFormatters[spec.argIndex]->format(out, spec, minWidth, precision);
+						} catch(const std::exception& e) {
+							throw std::runtime_error{"%s; when substituting format specifier “%s”"_format(e.what(), source)};
+						}
 					},
 					[&out](std::string_view text) { out(text); }
 				};
@@ -162,7 +178,7 @@ namespace format {
 							argCount = std::max(argCount, static_cast<std::size_t>(maxArg));
 							pos = newPos;
 
-							parts.emplace_back(spec);
+							parts.emplace_back(std::in_place_type<format_specifier>, spec, fmt.substr(nextSpec, pos - nextSpec));
 						}
 					} catch(const exceptions::incomplete_specifier&) {
 						throw exceptions::incomplete_specifier{"Incomplete format specifier “%s” at end of format string"_format(fmt.substr(nextSpec))};