CSV Writing Guide

This page summarizes write-side APIs and practical usage patterns for emitting CSV/TSV data.

Core Writer APIs

• csv::make_csv_writer()
• csv::make_tsv_writer()
• csv::DelimWriter

Use csv::make_csv_writer() for comma-delimited output and csv::make_tsv_writer() for tab-delimited output.

Writing Containers with <tt>operator<<</tt>

Any row-like container of string-convertible values can be streamed directly.

TEMPLATE_TEST_CASE("CSV/TSV Writer - operator <<", "[test_csv_operator<<]",
    std::vector<std::string>, std::deque<std::string>, std::list<std::string>) {
    std::stringstream output, correct_comma, correct_tab;
 
    // Build correct strings
    correct_comma << "A,B,C" << std::endl << "\"1,1\",2,3" << std::endl;
    correct_tab << "A\tB\tC" << std::endl << "1,1\t2\t3" << std::endl;
 
    // Test input
    auto test_row_1 = TestType({ "A", "B", "C" }),
        test_row_2 = TestType({ "1,1", "2", "3" });
 
    SECTION("CSV Writer") {
        auto csv_writer = make_csv_writer(output);
        csv_writer << test_row_1 << test_row_2;
 
        REQUIRE(output.str() == correct_comma.str());
    }
 
    SECTION("TSV Writer") {
        auto tsv_writer = make_tsv_writer(output);
        tsv_writer << test_row_1 << test_row_2;
 
        REQUIRE(output.str() == correct_tab.str());
    }
}

Writing Tuples and Custom Types

DelimWriter can also serialize tuples and custom types that provide a string conversion.

struct Time {
    std::string hour;
    std::string minute;
 
    operator std::string() const {
        std::string ret = hour;
        ret += ":";
        ret += minute;
        
        return ret;
    }
};
 
#ifndef __clang__
TEST_CASE("CSV Tuple", "[test_csv_tuple]") {
    #ifdef CSV_HAS_CXX17
    Time time = { "5", "30" };
    #else
    std::string time = "5:30";
    #endif
    std::stringstream output, correct_output;
    auto csv_writer = make_csv_writer(output);
 
    csv_writer << std::make_tuple("One", 2, "Three", 4.0, time)
        << std::make_tuple("One", (short)2, "Three", 4.0f, time)
        << std::make_tuple(-1, -2.0)
        << std::make_tuple(20.2, -20.3, -20.123)
        << std::make_tuple(0.0, 0.0f, 0);
 
    correct_output << "One,2,Three,4.0,5:30" << std::endl
        << "One,2,Three,4.0,5:30" << std::endl
        << "-1,-2.0" << std::endl
        << "20.19999,-20.30000,-20.12300" << std::endl
        << "0.0,0.0,0" << std::endl;
 
    REQUIRE(output.str() == correct_output.str());
}
#endif

Data Reordering Workflow

For read-transform-write pipelines, csv::CSVRow supports conversion to std::vector<std::string>, which makes it straightforward to reorder/select fields before writing.

Typical flow:

1. Read with CSVReader
2. Convert row to std::vector<std::string>
3. Reorder/select fields
4. Emit with CSVWriter

TEST_CASE("CSV Writer - Reorder Columns", "[test_csv_reorder]") {
    auto rows = "A,B,C\r\n"
        "1,2,3\r\n"
        "4,5,6"_csv;
 
    std::stringstream output, correct;
    auto writer = make_csv_writer(output);
 
    writer << std::vector<std::string>({ "C", "A" });
    for (auto& row : rows) {
        writer << std::vector<std::string>({
            row["C"].get<std::string>(),
            row["A"].get<std::string>()
        });
    }
 
    correct << "C,A" << std::endl
        << "3,1" << std::endl
        << "6,4" << std::endl;
 
    REQUIRE(output.str() == correct.str());
}