#include <QString>
#include <stdio.h>
#include <locale.h>

void printUnicode(const QString &str)
{
    for (int i = 0; i < str.length(); ++i)
        printf(" U+%04X", str.unicode()[i].unicode());
}

int main(int argc, char **argv)
{
    setlocale(LC_ALL, "");

    QString source = QString::fromLocal8Bit(argv[1]);
    QString composed = source.normalized(QString::NormalizationForm_C);
    QString decomposed = source.normalized(QString::NormalizationForm_D);

    printf("Source form:");
    printUnicode(source);

    printf("\nComposed form:");
    printUnicode(composed);

    printf("\nDecomposed form:");
    printUnicode(decomposed);
    printf("\n     categories:");
    for (int i = 0; i < decomposed.length(); ++i)
        printf(" %6d", decomposed.unicode()[i].category());

    printf("\n");
    return 0;
}

$ /tmp/decomposition/decomposition abcáé€.
Source form: U+0061 U+0062 U+0063 U+00E1 U+00E9 U+20AC U+002E
Composed form: U+0061 U+0062 U+0063 U+00E1 U+00E9 U+20AC U+002E
Decomposed form: U+0061 U+0062 U+0063 U+0061 U+0301 U+0065 U+0301 U+20AC U+002E
     categories:     16     16     16     16      1     16      1     28     26