All offspring from the cross between a fruit fly with straight wings (cc) and one with curly wings (CC) will have curly wings with a genotype of Cc. The ratio of dominant to recessive phenotypes is 1:0 as all offspring will show the dominant curly winged phenotype.
When crossing a fruit fly with straight wings (cc) and one with curly wings (CC), there are certain potential genotypes for the offspring. Since C is dominant over c, and the curly winged fruit fly has two dominant alleles (CC), while the straight winged fruit fly has two recessive alleles (cc), the resulting combinations in the Punnett square will all be heterozygous (Cc), displaying the dominant curly winged phenotype. Therefore, all offspring would have the same genotype of Cc, and thus all display curly wings.
For the calculation of the ratio of dominant to recessive phenotypes, since all offspring have curly wings, the ratio is 100% curly winged to 0% straight winged, or in simplest form, it's just 1:0, because no straight winged phenotype will show up due to the dominance of the C allele over the c allele.