avoid copy from reading file to string

Trying to figure out if there is a way to avoid this copy operation when reading a file into a string in any of the following ways.

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Copy made from calling t.str().

std::ifstream t("file.txt");
std::stringstream buffer;
buffer << t.rdbuf();
return t.str(); // Copy

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No copy operation here, but buffer does not contain the entire file after read.

std::ifstream t("file.txt");
t.seekg(0, std::ios::end);
size_t size = t.tellg();
std::string buffer(size, ' ');
t.seekg(0);
t.read(&buffer[0], size);     

This way is also pretty ugly with default-instantiating the string with size number of spaces.

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No copy, but contents are in char * and string(block) would perform a copy.

std::ifstream file(filename, std::ios::binary);
std::streambuf* raw_buffer = file.rdbuf();
char* block = new char[size];
raw_buffer->sgetn(block, size);
delete[] block;

Can't find a possible way to create string from char * without making a copy.